If R2 touches third base but misses home plate, how can he be retired?

The correct choice highlights a fundamental rule in baseball regarding the base running and the obligation of a player to touch all bases in order. When a runner, in this case R2, touches third base but fails to touch home plate, he becomes a vulnerable runner.

In order for the defensive team to retire R2, they can execute a simple yet effective strategy: if they step on the missed home plate, they effectively declare R2 out. This action fulfills the requirement that a runner must legally touch all bases in sequential order, including home plate, for the run to count. In this scenario, the missed base (home plate) acts as a point of failure for R2's run, allowing the defense to retire him simply by acknowledging that he did not complete the scoring process.

The other choices involve actions that do not directly address the requirement of touching home plate. For instance, a throw to first or tagging after the runner has scored does not rectify the missed touch of home plate. Thus, by stepping on the base that R2 failed to touch, the defensive team can successfully retire him.