Concept:For a choice between two people, “neither” means not one and not the other.
Explanation:Mary and Peter are only two people.
The word “but” creates a contrast with the speaker’s expectation.
“All” is used for more than two, so it is incorrect.
“Each” would mean both took the advice, which contradicts “but.”
“Either” normally means one of two, but the sentence needs a negative meaning.
Therefore, “neither” is the correct word.
It makes the sentence: I spoke to Mary and Peter on the issue but neither of them took my advice.
Answer:B. neither