Among Physics and computers science which is better as an optional subject for bsc Maths,Electronics
I must say computer science because in future there will be huge demand of computers, software engineer etc.
Computer science is problem solving and an essential skill of life.
Students study the design, development and analysis of software and hardware used to solve problems in a variety of business, scientific and social contexts.
Ask these combination will cover most of the basic knowledge required at your level for bsc maths, electronics.