They ask about the things and teachers ______ they still remembered in the school. A. which B. that

They ask about the things and teachers ______ they still remembered in the school. A. which B. that C. whom D. who

The correct answer is B. that.

The sentence would read:
They ask about the things and teachers that they still remembered in the school.

"That" is used to introduce restrictive clauses, which is the case here. It's referring to the specific things and teachers that the people remembered.

"Who" and "whom" are used for people, but "that" is more commonly used in restrictive clauses when referring to both people and things.