B.Sc

You can go for ITI apprenticeship else after ITI you can go for diploma as a lateral entry in a good college as well. Some information about apprenticeship that an apprenticeship programs include on-job-training accompanied by studies as well. As you have completed ITI, do not be confused with an internship. Unlike internships, apprenticeships are for the longer term, pay is high and you get a job after it. So its more like Job with salary and you get to work.

Hi, I have predicted the applicable colleges for you. From Shiksha:
Please check the list of colleges from the below link:
https://www.shiksha.com/b-tech/resources/ggsipu-college-predictor

You haven't said which exam, so here is a list of some:
https://www.shiksha.com/science/articles/top-bsc-msc-entrance-exams-in-india-blogId-13484
https://www.shiksha.com/nursing/colleges/b-sc-colleges-india.

12th is compulsary for B.Sc. Hope you know that.
Check this link and see prospectus. The application process started in May:
http://reg1.stellamariscollege.org/OnlineApp_Home.aspx

Hi Nitish,
Here is the link for cracking the UPCATET exam. It can help you get admission to B.Sc. colleges in India.
https://www.shiksha.com/science/exams/upcatet/question-
papers
All the best.

M.Sc. in Astrophysics will be better option to pursue. As you have already done Physics, so you will be eligible for it. Diploma will not be much helpful. Regular B.Sc. will have more benefits.