Greetings of the day dear aspirant

As you said you are appearing 12th improvement/ betterment exam this year, so you will fill code - 1 in NEET form.

CODE: 01

A candidate who is appearing in the qualifying examination, i.e., 12th Standard whose result is awaited, may apply and take up the said test but he/she shall not be eligible for admission to the MBBS or BDS, if he/she does not pass the qualifying examination with the required pass percentage of marks at the time of first round of Counselling.

You can have a detailed look at the NEET Qualification Codes from the below provided link.

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

Hope it helps!!

All the best for your future career