The best definition for the word "mortuary" that aligns with the meaning of the prefix "mort-" meaning "death" is "a tomb." A tomb is a burial place for the deceased. To find this answer, you can break down the word "mortuary" and recognize that the prefix "mort-" indicates a relation to death. By linking this understanding to your knowledge of vocabulary, you can deduce that a tomb is the most accurate definition.
The best definition for the word mortuary, keeping in mind that the prefix mort- means "death," is a tomb.