You have physics back ground so, your first choice should be physics but it is not always necessary to choose subjects of your back ground.

CSS syllabus of all subjects (2016 & onward)

http://www.cssforum.com.pk/syllabus.pdf

Firstly go through syllabus of physics. Analyse the subject syllabus. As you have subject background, you should already be familiar with 90-100% concepts in it. If that is the case, you can choose this subject. If you think your familiarity level with this subject is below 90% than you should not blindly think about choosing this subject because physics is not a subject which can be easily learnt at home. If you think you can cover rest of the topics with help of good teacher than that can be an option.

Now as Farhan Kaif has indicated, physics has been regarded as low scoring subject in CSS according to seniors. Many students with sound back ground of physics fears to take it as optional. Academies also suggest not to take this subject as optional if you want to score high. Many say that scoring trend is a myth well no it is not a myth. Some subjects are relatively high scoring than others but one can make them high scoring with exceptionally good preparation (Better than an average student do). I believe physics is a subject which can be turned into a very high scoring subject. Also not many attempt this subject and what I have observed the less the candidates the better the scores because examiner does not get many papers with same concepts written over and over again and also less papers to compare with each other.
During analysis of syllabus you should analyse your self as well regarding physics. One has masters degree in subject does not mean he or she is an expert in that subject. He or she might holds a degree in a subject but might be he or she has not learnt the subject that well in his or her masters and his or her sole purpose was to pass the exam merely. So, you should judge your self if you can handle the subject well or not as it is different from masters and you have to study a lot. Please do not think that I am saying that the ones who have good grades can attempt the paper. I am talking about paying attention and learning the subject in masters as scores in masters or previous degree does not matter here.
You have exactly 8.5 months left for CSS-2018 attempt. So, physics is 200 marks subject and you should at least give this subject one month for preparation.

One fact: Css 2016 Maleeha Lesrar had physics as her optional subject. Her score was 112/200. 66 in paper A and 60 in paper A which is a good score.

If you do not want to go for Physics than you need to check all subjects in group 1 and 2 and than you can choose 1 subject of 200 marks from group 1 or you can choose 1 subject of 200 marks from group 2 or 2 subjects of 100 marks each from group 2 (you cannot choose 1 subject of 100 marks from group 2 alone).

If you are not going for physics or any other subject which demands back ground than you have two options left which are in group 1 International relations & political science. You need to go through syllabus of these two subjects and will have to choose a subject which you find interesting. My suggestion here is international relations. It is more scoring and interesting. It can also help you with current affairs and essay.

You can also choose Physics and any one of the above mentioned two subjects (IR & PS) at the same time. They will make total of 400 marks if you choose them and rest you can choose two subjects of 100 marks each.

Now about rest of the subjects:
I have following recommendations:
1- Group 4 US history or Indo-Pak history (My preference US history and highly recommended if you are taking IR as your optional subject)
2- Gender 5 gender studies or environmental studies (Both are easy and interesting subjects (After score analysis of CE-16 result my recommendation is Environmental Science as it has turned out to be more scoring and also it will help you in compulsory subject of GS&A.)
3- Group VI-International Law (It is now very high scoring subject. It is also easy and can be easily prepared at home. It will also help in IR. It can be covered by consulting only two books)
4- Group VII- Psychology or sociology or regional language. All are pretty high scoring. My recommendation is psychology it is more interesting and it is scoring as well.

I have given you all possible best option now it totally depend upon your back ground, your interest and scoring trend of subjects in CSS as well. Make your choice wisely and not on the basis of single factor alone. It is most crucial decision regarding CSS and before making your final choice you need to go through syllabus of suggested subjects.

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