The CSIR NET JRF Exam is conducted by National Testing Agency for only science stream subjects which are -
So, you have to only choose that CSIR NET Subject in which you have pursued your psotgraduation.
Here you can check the CSIR NET Subject Eligibility.
If the CSIR NET Subject list is not matching with your postgraduation subject then you have to select the related subject from the list.
The difficulty level of all these subjects are different as per their syllabus.