What should I write as Class of Admission into the field 40.a. on Form I-130?

Class of Admission is a three-character code that is printed on your conditional or Permanent Resident Card (green card).

You can look up this code on your green card on the right of USCIS# number right under Category. For example, your Class of Admission may be B11, AR6, or R57.

USCIS assigns classes of admission to identify various visa categories that are used to admit immigrants to the United States.

 

Also read:
How do I replace a green card that has been lost, stolen or destroyed?