Question

diacetyl is...
...an ingredient in vape juice that some people also eat for a buzz.
...an ingredient in vape juice that is not good to inhale, but is not known to cause any illness.
...an ingredient that the fda has said is unsafe in any form.
...an ingredient that is considered safe in some foods to eat, but is not safe to inhale into the lungs.

Explanation:

Diacetyl is a compound. It is approved for use in food (so it can be considered safe in some foods to eat as it's used in food flavoring in small amounts). However, inhaling diacetyl (such as in vape juice) can cause serious lung problems like bronchiolitis obliterans (popcorn lung). The FDA has not said it's unsafe in any form (since it's allowed in food in proper contexts). And it's not something people eat for a "buzz". Also, it is known to cause illness when inhaled.

Answer:

...an ingredient that is considered safe in some foods to eat, but is not safe to inhale into the lungs.