You can use my car ________ you come back before Firday. A. unless that B. on condition that C. when

You can use my car ________ you come back before Firday. A. unless that B. on condition that C. whenever D. as if

Let’s break this down.

The sentence is:
You can use my car ________ you come back before Friday.

A. unless that – “unless that” is not a standard phrase in English. Normally it's just unless (no that). Plus unless means “if not,” which would give a meaning like: “You can use my car if you do not come back before Friday.” That contradicts the natural meaning here. So, not A.

B. on condition that – This is a correct formal phrase meaning “provided that.” It fits perfectly:
“You can use my car on condition that you come back before Friday.”
So B works.

C. whenever – This would imply “any time that you come back before Friday, you can use my car.” That would mean you can use it repeatedly on condition that you return before Friday each time, which sounds odd for a single event. But more importantly, “whenever” means at any time (repetition) or in a habitual sense, which doesn’t fit the sentence's likely meaning (one-time permission with a condition). So C is awkward at best.

D. as if – This is used for comparisons or unreal situations (“He acts as if he were the boss”). It doesn’t make sense here. Not D.

Clearly, B is the correct answer.

So the answer is B. on condition that.