B.Sc Botany

Yes, you can join for M.Sc geography.

He is eligible for M.Sc Biotechnology in well reputed universities like Nirma University, VIT-Vellore, and SRM- Chennai, if he passed entrance exam which is organized by these universities. Although, he can also apply for IIT JAM because some IIT institutes offer biotechnology courses and M.Sc. Ph.D. integrated courses, for those students who have completed their Science graduation without Maths.

According to your score and percentage the colleges under DU will be perfectly fine. So you can try applying under DU to get admission.

If you have good grades in 12th, you will easily get admission there. Just calculate your best 4 and check whether you are eligible for the courses by seeing the cut-off on the DU official website (or check if any entrance test is there for the courses you have mentioned, on the official website of DU).
The fee is very less since they are government colleges.

Apply for DU in June-July when registrations start. It offers B.Sc in Botany and B.Sc in Zoology.

Yes, you can. But, I don't think you can do all the three courses together, because geography doesn't belong to the biological side. But you can do Botany, Zoology and Geology. It will have high value.

You should do B.Sc (Hons) in Botany. It has good future prospects.

As per your background, I would suggest you opt for M.Sc in Biotechnology, whereas, it completely depends upon your interest. I have one of my classmate who did Bachelors in Biotechnology with me and then moved on for Mass communication for Master's degree because of his interest.

Which subjects are you interested in?

Yes, it is necessary for M.Sc to get a degree in the same subject. So, if you want to do M.Sc in botany you have to do B.Sc in the same.