Answer:
A
Step-by-step explanation:
If you put each word into that sentence, it will be clear that A is the answer. The first sentence, using A, will sound like this:
"People who eat too much popcorn risk getting kernels in their teeth."
Everything flows well together, and it sounds like a grammatically correct sentence.
The sentence using B will sound like this:
"People who eat too much popcorn risk getting kernels in his/her teeth."
This also flows well, but the reason this is not the answer is because we're not talking about one person, hence the word "people" in the sentence.
Sentence C would sound like this:
"People who eat too much popcorn risk getting kernels in them teeth."
This doesn't flow and isn't grammatically correct.
Hope this helped!