M.Sc

Course duration ranges from 2-4 years. Please refer to the link provided below for duration of part-time M.Sc of different colleges:
https://www.shiksha.com/science/colleges/distance-correspondence-m-sc-colleges-india

Please refer to the link provided below for details regarding admission in M.Sc (Agronomy) in Amity University, Noida:
https://www.shiksha.com/science/course/m-sc-in-agriculture-agronomy-amity-university-noida-232553

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.

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.

Qualifying the entrance exam itself may not make you eligible for a scholarship. If you have high marks in UG and in 10+2, combined with merit in the qualifying examination then you may be deemed eligible to receive a scholarship. Also, different universities have different criteria for scholarships. You should check scholarship eligibility of each institute individually.

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.

Some of the best private institutes for M.Sc in astronomy are as follows:
1) India Institute of Science, Bangalore
2) IUCAA, Pune
3) Punjab University, PU
4) Indian institute of Astrophysics, Bangalore

Eligibility for M.Sc in chemistry is under graduation in B.Sc with chemistry as one of the subjects, and maths as one of the subjects at 10+2 level. The minimum cut-off at undergrad level is 55% for general category and 50% for the reserved category. Candidates would become eligible for admission in the IITs after clearing the IIT JAM.