"Provided" is the only correct answer. As well as meaning "as long as", it also means "under the condition that".
Obviously, "although" and "unless" make no sense. "As" by itself is extremely unclear. Would it mean "because" or "while" in the sentence? Still, neither option makes sense.
There's nothing unusual or rare about any of the words given. You simply need to know them if you do a Cambridge test.