The ingredient in phenylethyl alcohol agar that supplies carbon and nitrogen is peptone.

Here's why:

* Carbon: Peptones are derived from the enzymatic digestion of proteins. Proteins are primarily composed of carbon, making peptone a rich source of this element.

* Nitrogen: Similarly, proteins also contain nitrogen, and peptone inherits this nitrogen content.

While other ingredients in phenylethyl alcohol agar might contain trace amounts of carbon and nitrogen, peptone is the primary source for these essential nutrients.