That is a Subject Question that we ask to find out about the subject of the sentence.
When what, which, who or whose refer to the subject, the question word comes before the verb without the use of the auxiliary verb.
If the answer is "The train to London was late." the question would be "Which train was late?"
If the answer is "I won the race." the question would be "Who won the race?"
More examples to refer to the present tense:
Which house belongs to Mr. Something?
Which is the house belonging to Mr. Something?
To refer to the past tense:
Which house belonged to him?
Which is the house that belonged to him?
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