I think friendly and playful work equally well in the first blank. But masked definitely works better in the second blank. The teasing may have seemed/appeared to be/come across as friendly or playful, but it masked/hid/obscured/disguised a long-standing hostility. Revealed has the opposite meaning. The teasing hid the hostility rather than reveal it or show it.

To me, "playful" seems like the best answer for the first blank, but I agree that "friendly" also seems possible. ("Aimless" would definitely be wrong, though.) "Playful" sound better to me because it's a word that's commonly associated with the idea of "teasing," which is a playful behavior. It's not necessarily a "friendly" behavior. The person who is doing the teasing is usually having fun ("playing"), but the person who is being teased may not think it's very friendly or nice. "Teasing" doesn't imply hostility, though. In fact, we usually use the word "tease" to show that the action is playful rather than mean. If your friends "tease" you because of your new haircut, it means that they're making fun of the haircut, but in a playful way, just for fun, not because they dislike you or want to offend you. (Of course, you may still feel offended. Just because your friends are being playful doesn't mean you will necessarily be happy about it.) In the second part of the question, only "masked" seems correct to me, due to the words "but in fact." These words tell us that the teasing was not what it "seemed" to be. If you say "revealed," then it sounds like the teasing was visibly or obviously hostile -- it revealed or showed hostility. It makes more sense to say that this behavior was *secretly* hostile, even though it "seemed" to be playful.

I suppose "friendly and playful" are equally well in the first blank. Although, "masked" definitely works better in the second blank.

It’s hard without seeing the rest of questions/test or what it was asking. But aimless is the best choice because in the first part you’re implying there is no purpose or reason for the teasing. Then second part of sentence reveals that actually there was purpose to why their teasing seemed aimless and that was to mask their hostility.

Let’s start with the second blank first. The word “revealed” means that the first clause shows something that LEADS to the second clause. “Masked” means that the first clause is HIDING whatever the second clause is explaining. Now let’s go back to the first blank. The fact that something was “masked” means that the first thing must be the opposite of the second thing. “Hostile” is the opposite of “friendly.” Hope that helps!