Hiii

For the Reason assertion you should first of all study NCERT thoroughly because most of the time these questions are based on the lines of ncert itself. Then you should get a good book which would have previous year questions and sample questions. Keep on practicing reason assertion questions until you excel it and become comfortable with any type of question.

Here is a link for aiims best books. you can use it for your help.
https://medicine.careers360.com/articles/best-books-for-aiims-mbbs

Hope this is helpful

Thanks!!!!