You didn't said about the subject combination of your graduation. Now as per your question you can use any of the three codes based on your case:

• Code 3: An aspirant can choose this code if the aspirant have passed Intermediate/Pre-Examination degree in Science stream from any recognized Indian University/Board or other body with Physics, Chemistry, Biology/Biotechnology, and English as a compulsory subject. The qualifying examination should also include a practical test in the mentioned subjects.
• Code 4: An aspirant can choose this code after the completion of pre-professional/pre-medical examination with Physics, Chemistry, Biology/Biotechnology, and English after passing either the Higher Secondary examination or Pre-university or equivalent exam. The qualification of pre-professional or pre-medical examination should compulsorily include the practical tests in all subjects.
• Code 6: Any aspirant who has passed B.Sc. examination with not less than two subjects which includes Biology/Biotechnology, Physics and Chemistry can choose Code 6 from NEET qualification code 2021 as their eligibility code. Further, such candidates must have passed Class 10+2 qualifying examination with English, Chemistry, Physics and Biology/Biotechnology.

Now if you don't have this kind of combination in your UG then you have to use the code 2 as you will be accounted to be a dropper.

The code 2 is for those students:

• Completed 12 years of school education from CBSE/ICSE or any other boards with the last 2 years having the subjects English, Physics, Chemistry, Biology/Biotechnology, or Mathematics or any other elective as a major subject .
• Students who have passed from NIOS/other open schooling board with Biology/Biotechnology as one of the subject in 10+2.
• For the droppers.

To know more about NEET code visit the link below:

https://medicine.careers360.com/articles/neet-qualification-codes

I hope this answer helps. All the very best for your future endeavors!