He tried to remember the names of all the people ______ were there that night.

The correct answer is "whom" because it functions as the object of the verb "remember" in the relative clause. In this case, the phrase "the names of all the people whom were there that night" indicates that the individuals being referred to are not the subjects performing an action but rather the ones being remembered.

In standard English, "whom" is used when referring to the object of a verb or preposition. Although in casual conversation or informal writing, people often say "who" instead of "whom," in more formal contexts such as the one presented here, "whom" is appropriate.

While "that" can also be used in reference to people in informal contexts, "whom" is more precise when identifying individuals as the object of a verb. The options "whose" and "which" do not fit in this context; "whose" indicates possession, which is not relevant here, and "which" typically refers to things or animals, further making them unsuitable for referring to people.