Answer:
Your answer would be B.
Step-by-step explanation:
In order to understand the difference between who, whom, and whose you first have to understand the difference between subjects, objects, and possessives.
Who is an subject pronoun. It is used to indicate ask which person did a certain action or which person is in certain state. On the contrary, whom is an object pronoun: it is used to ask which person received a certain action. Finally, whose is a possessive pronoun which is used to ask which person something belongs to.
Taking all this into account, it is possible to say that B is the correct answer because the question indicates that someone is receiving the action (someone visited someone else). In terms of grammar, whom is the correct one. However, as this version is falling into disuse save for written style, it is also okay to use the subject pronoun who in this case. Yet, B is the right answer in terms of grammar.