6.
The answer is: A
Explanation
The correct option is A: upon.
The phrase "prevailed upon" means to persuade or convince someone to do something. In this context, it means that after a long discussion, you managed to persuade him to accept the proposal.
The other options do not fit grammatically or contextually:
"Against" implies opposition, which does not fit the intended meaning.
"To" is not used in this context.
"Above" does not make sense in this context.