"Let's cook the taro."
Translation:E hoʻomoʻa kākou i ke kalo.
My dictionary refers to moʻa, which means cooked or burned, as by the sun. Moʻa acts as the stem, and hoʻo- is a common prefix before (stem) words beginning with i, u, and all consonants except the ʻokina. It suggests causation. Kuke also means cook or to cook as one of its meanings. It sounds so much like "cook" that I suspect it is a word borrowed from English, but thatʻs just a guess.
So I tried to look up "ho'omo'a" in the Duolingo dictionary (since I thought both of the other translation choices (which used "kuke," which DOES mean "cook") were incorrect for other reasons) - and the dictionary said "There doesn't appear to be a translation for "ho'omo'a" in either of the selected languages. Try another search?" So I didn't select this translation (the "correct" one, i.e., "Translation: E hoʻomoʻa kākou i ke kalo.") So ho'omo'a needs to be in the dictionary.
PS after reading the other comments here, I see that "ho'omo'a" comes from "mo'a," so I looked that up in the Duolingo dictionary too and got "There seems to have been an error. Please try again." So "mo'a" needs to be there too.
(I realize there are other dictionaries, and "mo'a" is in http://www.wehewehe.org/, but seems if it is used in Duo it should be in the Duo Dictionary.)
In my experience, Duolingo is not meant to be the sole source for learning the language, which you have obviously figured out. Whenever I see ho'o, my first thought is that it suggests causation. I'm sure it's not so simple, nothing is, but that's a start. You're right that mo'a means cook, so the sentence above makes sense = "Let's all (three or more people) [do] cook." If there is a takeaway in my comment, it's that "ho'o" is a prefix for causation. :o)