"Are they cooking for me?"
Wananipika would be "they are cooking me" (or since this is a question, "are they cooking me?"). Changing it to the prepositional form, wananipikia makes the action to, at or for someone, "are they cooking for me?"
If you're using the app, I strongly recommend visiting the website to read the tips & notes for the various skills.
Update - In regards to your suggestion of me reading the tips & notes, I do. I thought that perhaps I just forgot, but after moving on I realized that the notes explaining "wananipikia" is under "Prepositional verbs," which comes sometime after the lesson where this question is.
Maybe the staff at Duolingo might want to include a short introduction to 'prepositional verbs' in this lesson so that this question makes sense...? Otherwise it's confusing to introduce grammar points ahead of time :/
Ah, yeah. I assumed the question was from Prepositions (since I came here viewing the sentences page in Swahili discussion, rather than a lesson or strengthening).
There's other prepositional information in the tips & notes of Reciprocal as well. Sometimes I find myself going back and forth between several skills to refresh my knowledge of just one.