Infield fly rule applies when:

The infield fly rule is designed to protect the offensive team by preventing the defense from taking an unfair advantage in certain situations. This rule specifically applies when there are fewer than two outs, and there are runners on first and second base (or bases loaded). When these conditions are met, a fly ball that is not caught by the infielders raises the risk of a double play, as runners may hesitate thinking the ball is going to be caught. By calling the infield fly, the umpire ensures that the batter is out, and the base runners can advance at their own risk, eliminating the chance for an easy double play.

In this context, the other options do not satisfy the requirement for the infield fly rule to apply. For example, if there are two outs (as in the second choice) or no runners on base (as in the last choice), the protections offered by the infield fly rule are not necessary or applicable. The essence of the rule lies in its ability to address specific gameplay situations where the defense may exploit the circumstances to the detriment of the batting team. Thus, the correct answer accurately describes a scenario where the rule is active and intended to maintain fairness in gameplay.