Answer:
The transaction is completed on behalf of the Rep Payee because he/she controls the funds in the account. It is completed on behalf of the beneficial owner of the funds because, well, he/she is the beneficial owner. And of course, the Rep Payee is the individual named in Section B (by checking off "conducted on own behalf"). Take a look at this old (1995) Q&A on CTRs and scroll down to Question 23 for a parallel situation.
First published on BankersOnline.com 12/15/08