Optional subjects should be selected keeping in view certain things. First of all, you must have in view your academic background. Give more preference to those subjects which you have studied during your masters or bachelor programme. Secondly, it is appropriate to select those subjects for which you have natural tendency and aptitude. For example, you have done masters in English, so you may have more inclination towards philosophy and literature. Or it is also possible that you feel more pleasure in studying any other subject. I have done masters in chemistry. So, I am more inclined to scientific subjects. Hence, it is better to justify that what subject one can study with more interest.

Another important thing to keep in mind while selecting the subjects is to justify scoring potential. It is true that few subjects have more scoring trend than others. psychology, sociology and regional languages are more scoring than other subjects. However, I have not attempted css yet, so could not tell you that with assertion. It is clear that general subjects are capable of gaining more marks than others..

I will recommend you to select psychology/philosophy, sociology, journalism. Other subjects you may choose yourself.. I will be glad to tell you more things if you are interested to ask. Moreover, I am sending you a link you must check it. Here you may find some more information about your query..

http://www.cssforum.com.pk/beginners...-strategy.html
Stay blessed..