Last play penalty question and enforcement

[kfo9494]

Had something similar to this the other day at a practice srimmage. And we talked about it but never felt comfortable about the answers--

Game tied and A has the ball at midfield trying to score on the last play of the game.
At the snap A only had six players on the line of scirmmage. Then as the QB is dropping back he fumbles the ball. B78 pickes the ball up and runs to the 5 yard line. During the run B32 makes an illegal block at the 15.
Of course B wanted the decline the penalty and attempt a field goal with an untimed down. They send the field goal unit on the field.
As the ref was giving the option to A, they declined the penalty for the block. This would result in NO untimed down. Now B is wanting to take the penalty so the period would be extended.

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My question is-- What is the best way for the officials to handle this type of situation? Do you let the captains know every type of situation before excepting? Do you say, Let's find out want A wants to do first?