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.
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."
The dictionary defines inu as:"nvt To drink; a drink, drinking. Kāna mea inu, his beverage. Kona inu, his drinking."