Question

"Eating flesh with razor-sharp teeth" is how piranhas survive.

prepositional phrase
gerund phrase
participial phrase
appositive phrase

A?

Answer 1

"with" is a preposition, so "with razor-sharp teeth" is a prepositional phrase.

But what is "Eating" in this sentence?

Answer 2

I originally thought that was a gerund but isn’t “eating” a verb?

Answer 3

Gerunds are verb forms, yes, and they act like nouns.

Yes, it's a gerund phrase -- the whole phrase is the subject of the sentence.