B. Theres a red gate at the park's entrance, contains a contraction that is formed incorrectly.
This is because, there's nothing like theres!
There's always a "There's" and not "Theres".
"There's" simply means "There is or There was" in some cases.
There's a contraction "it's" and "its", both are correct.
But for "There's" there's another word i.e. "Theirs"! Which is the right word.
So B. Should be There's a red gate at the park's entrance.