They stayed for hours,_____ my mother was very annoyed about.

EASY ANSWER

The relative pronoun “which” after a comma is used to replace the whole clause before the comma.

The remaining answers:

A. this- demonstrative pronoun, meaning “this, this”.

B. whom- replaces a noun indicating a person, acts as an object.

C. that – A relative pronoun (ĐQH) replaces a noun indicating a person, thing or a whole cluster of people and things, acting as the subject or object. “That” is used only in defining relative clauses and is never preceded by a comma. Forced to use “That” in cases with modifiers indicating superlatives and absolutes such as The biggest…/The only/The last/The first, the second../etc.

They stayed for hours, which made my mother very angry.