Hey there!
Electors aren't required to cast ballots for the candidate to which they pledged. This is your answer.
The constitution says that electors gather in their respective states, so it can't be C), and some of the electors shouldn't be inhabitants of those states so we can knock off D), there is no requirement for their character so not E), and members of congress cannot participate as electors so it isn't B), this leaves us with answer A).
I hope this helps! :)
