Science

Agriculture by itself will be your major while pursuing M.Sc agriculture. During the course, you may be required to select subjects within the course, which will entirely your choice. You should select the subject you feel most comfortable with.

Courses you are eligible for:
1) B.Sc. (Computers)
2) B.Tech (CS)
3) BCA

M.Sc in chemistry eligibility:
1) B.Sc. degree after 10+2 with majors/honors in chemistry.
2) B.Sc. (General/Pass) with chemistry as one of the main subjects.
3) At least 50% marks at under-graduate level for general candidates and 455 marks for SC/ST candidates.
4) Candidates may also be required to clear an entrance test like the IIT JAM to qualify for the IITs or NITs.

If you want to make your career in the teaching field then B.Ed is a good option.

Please refer to the link provided below for a list of colleges:
https://www.shiksha.com/science/colleges/biotechnology-colleges-india

These are two completely different fields. One leads to management while the other places you in the technical field.
So, first, you have to decide between the two. B.Sc is a good option if you are planning to pursue a career in the scientific field or planning to do a Masters in science.
BBA is mostly opted by commerce students who have an inclination towards the management field. Scope for students opting for a BBA is high who can then go for an MBA and get employed as managers in the administration of firms.
Both have the option to join defence services through the CDS exam or civil services.

I don't think so. As far as I know, one must complete B.Tech to get admission in any country.

No, it is not. After completing B.Sc, you are free to opt for a career of your choice or opt for higher studies, whichever you deem fit.

Different colleges have different admission criteria. Normally, candidates should have the following qualifications to be eligible for B.Sc.
1) Students with 50% at 10+2 level from a recognized education board with science stream can opt for B.Sc.
2) Some institutes also conduct entrance exams for admission into B.Sc.