Thanks, johnnyG and OoTmaster.
Is it that both "at" and "by" would be possible in (b)?
In that link I posted, it seems that the deciding factor is whether the speaker (and people like me, who are looking at this sentence) are seeing it as a passive construction, or something else (that closely resembles a passive construction, but is also quite similar to other expressions/phrasings that are not passive).
I see (b) as a passive, so I chose
by for the blank:
--The result of the game excited him.
--He was excited by the result of the game.
If I use "got" instead of the usual passive version of "to be", it sounds better:
He got excited at the result of the game.
("The result of the game got him excited") < not a passive
The sentence pair for that would be something like:
The result of the game caused him to get excited.