Answer:
Metaphor
Step-by-step explanation:
A metaphor is a figure of speech that compares two things by saying the first thing is the second thing. Greta is obviously not actually a fish but she swims like one so the author used a metaphor.
WHY THE OTHER ANSWERS DO NOT WORK:
A pun is a joke exploiting the different possible meanings of a word. There is no joke being made here so the sentence is not a pun.
Personification is a form of metaphor in that human characteristics are attributed to nonhuman things. Greta is a human so this sentence is not an example of personification.
A simile is a figure of speech that compares two things using the terms "like" or "as". Neither of those phrases were used, so this sentence is not a simile.
The only option left is metaphor. :)