Mathematics

Here are the combinations:
Maths + physics + chemistry
Maths + Defense studies + physics
Maths + chemistry + computer science
Maths + physics + geography
Maths + geography + chemistry

Please visit the link for details: https://www.shiksha.com/science/course/m-sc-in-mathematics-school-of-distance-education-university-of-kerala-thiruvananthapuram-298822

Questions based on physics, chemistry and mathematics can be asked. A basic English Written Test can also be taken.

Yes, there are many job opportunities after M.Sc and you can even go for teaching profession.

Yes, you are eligible for M.Sc. Many universities offer this course. One of the finest universities of India, Delhi University, also offers this course. Admission for this course is based on the entrance exam. IIT also offers the post graduation programme M.Sc Maths. So be prepared for the entrance exam. But some colleges also offer this course on the basis of your marks in B.Sc Maths.

Yes, you can apply for M.Sc Mathematics.

Eligibility for M.Sc in Astronomy:
*The minimum eligibility required to be met for admission to the course is B.Sc or an equivalent degree with a minimum aggregate score of 50%.
*A few universities and colleges in India conduct entrance examinations for admission to the course.
Good luck.

https://onlineadmission.ignou.ac.in/admission/StudentRegistration.aspx
Click the link given above to apply for M.Sc in IGNOU.
Good luck.

Hi Arnab,
No, there will be no disadvantage. If we talk about Maths, it is not an easy sub for PhD. You will have an advantage over others that you have core knowledge of B.Tech, and this can help you in the research work after PhD.