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

prepositional phrase
gerund phrase
participial phrase
appositive phrase

A?

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

But what is "Eating" in this sentence?

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

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.

It seems that the phrase "Eating flesh with razor-sharp teeth" functions as a participial phrase.

To determine and confirm this, let's break down the options and their definitions:

1. Prepositional phrase: A phrase that begins with a preposition and usually acts as an adjective or an adverb in a sentence.
2. Gerund phrase: A phrase that starts with a gerund (a verb acting as a noun) and its modifiers or objects.
3. Participial phrase: A phrase that begins with a participle (verb form that acts as an adjective) and its modifiers or objects.
4. Appositive phrase: A phrase that renames, identifies, or modifies a noun or pronoun.

Looking at the phrase "Eating flesh with razor-sharp teeth," we can observe that it begins with the participle "Eating." This participle acts as an adjective describing the noun "piranhas." Furthermore, it has its own modifier, "with razor-sharp teeth," providing additional information.

Based on this analysis, we can conclude that "Eating flesh with razor-sharp teeth" is a participial phrase that describes how piranhas survive by using their razor-sharp teeth to eat flesh.