Yes, you are eligible for the BMAT, i.e. the BioMedical Admission Test as you have opted for the science stream (Physics, Chemistry, and Biology). But you can only be eligible for this test if you pass the 12th standard with the same Physics, Chemistry, and Biology along with Mathematics. So, you can give the BMAT exam after completion of your 12th standard.

Hope this has cleared your doubt. Thank you.