Actually syllabus is quite huge but I will mention the link where you can get all the information about the syllabus.

https://www.google.com/amp/s/competition.careers360.com/articles/upsc-ias-syllabus/amp you can follow this link to get all the information about the syllabus.

Coming to the optional subject then I would suggest you to take Anthropology or Sociology. These two are one of the basic subjects in your Mains optional and chances of change in syllabus is also quite less.

All the best!!