Science

You need to have at least 50% marks for admission in M.Sc Physics.

No, it is not affiliated to Tribhuvan University. Please visit the link to know about the affiliations:
https://www.dpmc.in/affiliation.html

Well, the better option is for Microbiology, you need to note one thing check the best possible option from the universities like Amity university, Lovely Professional University, SRM, placements are very good in these universities.

Admission to M.Sc programmes at NIT, Warangal will be based on JAM 2018 test being conducted by IITB. Hence the aspirants for M.Sc. programme at NIT-W are required to register and take part in JAM 2018. Admissions to M.Sc programs in mathematics, chemistry and M.Sc. (Tech.) engineering physics are done through rank obtained in JAM 2018 test being conducted by IIT, Bombay. The following specializations are offered in these programs. M.Sc. (Mathematics) with specialization in
Applied Mathematics and Mathematics & Scientific Computing and M.Sc (Chemistry) with specialization in Organic Chemistry and Analytical Chemistry.

Yes, you can migrate after one year to another college of DU. But the migration mainly depends on the cut-off released by the particular college according to the seats remaining vacant. If there are no seats vacant in that particular college, then you will not get admission. And the cut-off is on the basis of your 1st-year marks. The cut-off is always high.