How should an umpire rule if R3 interferes while following a bunted ball that results in a fly ball?

When R3 interferes with a bunted ball that leads to a fly ball, the umpire should rule that R3 is out for interference. This is based on the principle that a base runner is responsible for avoiding collisions with fielders making a play on the ball. When R3 interferes, this act disrupts the defense's opportunity to make a play, and the rule is designed to maintain fairness in the game by holding the offending runner accountable.

The interference typically results in an out being called on the runner who caused the disruption, which in this scenario is R3. This ruling is aimed at preserving the integrity of the game and ensuring that defensive players have a reasonable chance to make plays without being hindered by base running actions.

Consequently, the other options do not apply because: the ball would not remain live thanks to the interference; B4's status at first is not determined by the occurrence of interference, and F5 would not receive a free base simply based on the act of interference that occurred with R3. Hence, the correct response in this situation reflects the consequences of the interference committed by R3.