Now and once both can be adverb and conjuction. The blank should be conjuction because there are two phrases in a sentence. As conjuction, 'once' can be used If something happens once another thing has happened, it happens immediately afterwards. And 'now' can be used to indicate that an event has occurred and as a result something else may or will happen. In this shown sentence, the context says the lease is not approved yet, but IF it's approved, the tenant will be able to ~~~~. I think you are Korean so if I explain in Korean: Once means 일단 ~~하면 ~~할것이다. Now means ~ 이제 ~~했으니/하니

If you were to use 'Now', it would have to be 'Now that'. 'Prospective' can be replaced with 'future' (Though it doesnt have exactly the same meaning), which means that he's not yet the tenant of the new apartment. Also, 'will be able to (...) move in" indicates that he isn't able to move in yet, but he will be in the future. Using 'Now' instead of 'Once', the sentence would look like this: Now that the lease is approved, the new tenant is able to officially move in the new apartment.

Basically, to use "now", you would have to say 1) "Now THAT the lease is approved..." rather than 2) "Now the lease is approved". I think that 2/b is wrong because it sounds like its own sentence, rather than a phrase explaining the rest of the sentence about the tenant. Hope this helps!! If you need further explanation, let me know ^_^