Hello aspirant!
Hope you are doing well!

There are the set of parameters which the candidates must have fulfilled to appear in the medical entrance exam, that are laid down by National Testing Agency, the competent authority responsible for conducting NEET yearly

One of the most important and relevant criteria is

• Candidates must have passed 10+2 or equivalent with Physics, Chemistry, Biology/Biotechnology and English as core subjects from a recognized board.
  
  Hence biology must be a core subject, since you are looking forward to make a career in field of medicine dealing typically in science- biology.
  For more information refer to https://medicine.careers360.com/articles/neet-eligibility-criteria