The subject selection for the UPSC exam depends on your choice. It doesn't matter from which background you are. For the civil service exam (IAS/IFS/IPS), you have to choose only one optional, and for IFoS, you have to choose 2. If you are an engineering graduate, Mathematics will be a good optional for which you might have the pre-requisite knowledge. But you should select Mathematics only if you have an interest in that subject. Always remember that coaching for Mathematics is not widely available compared to other subjects. If you think you can prepare the subject by yourself only then opt for it.
If you don't feel confident in Mathematics, you should focus on the big four art subjects (History, Geography, Polity, and Economics).
Select one subject you find interesting, and feel the confidence you can write the answers, refer to previous year question papers. The biggest advantages of these subjects are that you can find many trainers and study materials who can guide you. Find someone who can guide you. It can be any coaching institute or anyone from your friends or family.
Besides these subjects, Public Administration is also a good choice. Don't go for engineering subjects like Mechanical, Civil or Electrical if you don't feel confident. These subjects are lengthy, took a lot of time.
