Box A will be "Income", box B will be "1040" (as long as you are referring to an individual and not an entity return), box C will be the years that you want transcripts for, and box D depends on why you need them. Below are the 8821 instructions for box D:
Enter any specific information you want the IRS to provide. Examples of column (d) information: lien information, balance due amount, a specific tax schedule, section 4980H employer shared responsibility payment information, or a tax liability. Enter “not applicable” in column (d) if you are not limiting your appointee's authority to inspect and/or receive all confidential tax information described in columns (a), (b), and (c).
For requests regarding Form 8802, Application for United States Residency Certification, enter “Form 8802” in column (d) and check the specific use box on line 4. Also, enter the appointee's information the same as instructed on Form 8802.
Thank you so much for this answer! If you only write 1040 in column b, does that also cover other forms that are also likely to be on an individual tax return, e.g., Form 8949, 1116, 8880, 8995, etc., or do you have to list every tax return form that has its own number?