I was taught mea inu for drink as a noun. So the word mea is optional then?
"mea" also means "thing"...so adding the mea to "mea inu" does turn it into a noun, but just "inu" is correct as well. Although, "mea inu" is more frequently used.
The dictionary defines inu as:"nvt To drink; a drink, drinking. Kāna mea inu, his beverage. Kona inu, his drinking."
I think "mea inu" is a noun, whereas "inu" is a verb. In my head, I translated the phrase to be "This is drinking" or "This is how it is to drink." Maybe the prompt should have been "He mea inu keia."