From Group III, I don't advise opting P.Ad. because of the reason you yourself have mentioned. This is going to become the new Journalism of CSS lol (where you get a wave-like scoring trend making it all look like a gamble)

As for GPP and TPUM: I have gone through the syllabi of both. Both have their pros and cons, I am spelling them out here and then you can easily judge and decide.

GPP is like an amalgam of sociology and political science. It can help you with your compulsories because many topics contained therein are kind of general ones. This will also mean that its material would be easy to find and easy to read. But this general nature of its syllabus also has its downsides. For example the examiner can complicate some of the questions for which you may not have prepared. Another bad thing about it is the length--it is kind of lengthy, regardless of it being simple, it is going to consume time.

On the other hand TPUM is a bit technical. But not as technical as to discourage a dedicated aspirant who has no background in it. Its biggest advantage is that it has a shorter length. It will take lesser time to prepare. But its bad side is that the examiner can get too technical and aspirants who have no background may feel short of material to produce an adequate answer.

Taking all of these factors into consideration, let me tell you what I think. New subjects--whether it be GPP, TPUM or GS--are going to fare better simply because people are not opting them this year to stay safe. Secondly, the examiners of these new subjects would like to encourage the aspirants to opt these subjects in the future by being initially lenient. Thirdly, examiners of social science (like Socio, Journalism and Psychology) always are lenient UNLESS they are asked to be otherwise. Socio and Journalism used to be that hen which lays golden eggs for CSS aspirants. This changed when FPSC commanded the respective examiners to be strict so as to discourage those who had started to opt these by the thousands!

If I were you, I would opt TPUM if I were short on time and GPP if I had ample time on my hands.

As for G.S, even though people are getting attracted to it, but I am still sure that the proportion is not as high as you have estimated. It is still a new subject and people avoid new things even if they are attractive. Because you have a non-biology background, you are not going to have any other option from Group V except G.S. So, I think if you are left with no other option, opting this becomes a matter of compulsion.

As for the histories, I think all of them are going to take equal amount of time to prepare. But as for the scoring trends, British and US history have been better than the others. But I am predicting that US history is going to come down now because everyone is going after it. If I were you, I would choose US history if I had short time and would choose British if I had ample time.

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"Everything the light touches, is our kingdom."