Situation: bases are loaded, the batter hits a single to right fielder, runner on third base scores then runner on second, runner on first and the batter-runner all score on the error made by the RF who doesn’t catch the ball that continues its trajectory towards the deep outfield. 4 runs are scored on the play.
Probably not B), as the runner from first surely does not score “unaided by an error.” (Even when Enos Slaughter famously scored from 1st in the 1946 World Series, the scorer gave batter Larry Walker a double.)
You have to make a judgment if the runner on second would have scored without the error (quite likely on a single to right field) or not. So your picks are:
A) single, E9, RBI (runner from second would NOThave scored without the error)
B) single, #9, 3 RBI (runners from first and second would have both scored even without the error)
C) single, E9, 2 RBI (runner from second would have scored even if the right fielder had fielded the single cleanly).
I believe the convention would be to give the benefit of any doubt to granting the second RBI.