"Have a good afternoon Esther"
Translation:Mchana mwema Esther
In the mulpiple choice question, you should now give both answers as a correct translation, so possibly you are correct. Saying Good afternoon seems to me a bit unnatural in English, though it is very common in Dutch. The Dutch word voor good afernoon is "goede middag" which is shorter to say I guess.
As a greeting, "good afternoon" is not unnatural. But "Mchana mwema" is not really a greeting. As in you don't lead off with it in a conversation. It's more for at the end of a conversation when you would say like "ok, well have a good afternoon" in which case saying "good afternoon" (by itself) would be unnatural.
You will learn more about this later in the course. It depends on the noun class. e.g. siku is in the N/N noun class and then you need to use njema, fas well as for asubuhi and jioni. You need mwema for mchana and usiku. You just need to learn the noun class for each word. The bad news is that in this course noun classes are not explicitly mentioned, so you will be regularly surprised by the actual noun class. Noun classes are important in Swahili, almost everything depends on it, so be prepared for a lot of confusion.