Answer:
Step-by-step explanation:
Moaning Myrtle's sneakers light up when she runs.
There are a couple of errors in the sentence. Moaning is an adjective describing a particular Myrtle (one that moans).
Myrtle is the one owning the sneakers. Normally Myrtle would be a noun. It isn't. It's a possessive noun. Myrtle is a possessive and requires an apostrophe.
sneakers is the answer.