After a fly ball to F8, if F8’s throw retires R2 for the third out while R3 has already scored, what happens?

In this scenario, when R2 is put out by F8's throw while R3 has already scored, R3's run counts, and R2's out stands. This is because R3's run is considered to be completed before the third out is recorded. The timing of plays in baseball is crucial; once a runner crosses home plate and the run is scored, it is valid as long as that runner was not the one responsible for creating a force out.

In this case, since R2 is the one who is put out, it does not negate R3's prior run. R3 scored before the out was made, so the score will count. This aspect of the play illustrates how the rules of baseball prioritize the sequence of events regarding when runs are scored in relation to outs being made. Thus, the correct answer encapsulates the idea that R3’s successful crossing of home plate is unaffected by the subsequent out of R2.