Which is the simple subject of the sentence? Five fuzzy chicks walked around the coop.

Chicks. The sentence is about what the chicks did.

"Chicks," is the answer, because a simple subject tells us who is performing the action. When looking for the simple subject, leave out any modifiers or descriptors (e. g. "five fuzzy") because you want the subject to be simple. I hope that makes sense!

Here's an example: "The bright blue bird landed on a nearby tree." The simple subject is, "bird." ("bright blue" is a modifier/descriptor and is not a part of the simple subject)