This house is used very rarely by us.

Answer: Option B

The given sentence is in the passive voice: "This house is used very rarely by us."

To convert it to active voice, identify the subject, verb, and object:

Subject (doer): us (which becomes "we" in active voice)
Verb: is used (present simple passive)
Object (receiver): this house

The present simple passive ("is used") changes to the present simple active ("use") in the active voice.

So, "This house is used very rarely by us" becomes "We use this house very rarely."

Therefore, Option B is correct.