The only way to be sure to get the correct answer is to sneak a peek at the hint, which is less than ideal for the lesson. Hopefully enough people will report it that they get this glitch cleared up. In the meantime, we need to remember this is still a beta, so we're bound to find those glitches. It can be maddening, but... We're at least getting to learn some Finnish.
This is what I also thought at first, but it can mean "one coffee/a cup of coffee", and then it's a countable noun. @Dasshutsu explained it to me well here:
Pretty much, but it's definitely more common with verbs. In spoken Finnish you'll often hear the "ko/kö" left out of questions like this that aren't complete sentences. However, you absolutely can stick it to the end of all kinds of words:
"Anna se minulle." "Tämäkö?" Or "Tämä kirjako?"
"Give me that." "This?" Or "This book?"
"Tämä on suuri." "Suuriko? Eikö tämä ole aika pieni?" "This is big." "Big? Is this not quite small?."
So here you have examples with the interrogative "ko/kö" at the end of a pronoun, a noun, an adjective and the word "ei" which behaves like a verb in Finnish.
You can use it with most types of word I think, at least informally:
Verb: Juoksetko? - Do you run?
Noun: Kahviko? - A coffee?
Adjective: Suuriko? - A big one?
Numeral: Kaksiko? - Two? (As in "was it two you wanted?", for example)
Adverb: Nopeastiko? - e.g. "(did they do it) quickly?" or "was it quick?"
Nominative: Nainenko? Mieskö?
Genetive: Naisenko? Miehenkö?
Partitive: Naistako? Miestäkö?
Essive: Naisenako? Miehenäkö?
Translative: Naiseksiko? Mieheksikö?
Inessive: Naisessako? Miehessäkö?
Elative: Naisestako? Miehestäkö?
Illative: Naiseenko? Mieheenkö?
Adessive: Naisellako? Miehelläkö?
Ablative: Naiseltako? Mieheltäkö?
Allative: Naiselleko? Miehellekö?
Abessive: Naisettako? Miehettäkö?
(Plus the plural forms.)
For those overwhelmed by this (since for some reason I can't respond to those comments directly), don't stress about learning all the noun cases immediately.
Even at an intermediate level, it's enough to recognize most of them, especially the few which are not used very often. You don't need to be able to use everything right away. As a beginner, focus on the genitive, partitive and accusative at first (and the accusative just looks like either the nominative or genitive), then as you move on to intermediate stuff, the locative cases. Then the essive and translative are still pretty common.
This page may help break down the mass of 15 cases into more managable chunks: https://en.wikipedia.org/wiki/Finnish_noun_cases
I don't think that a full "thank you" is correct here. If the sentiment of the question is "would you like a coffee?" then the answer would be "yes please" or shortened to "please" or the more casual "thanks" as an answer to the question. A full "thank you" would be used after the coffee was actually delivered.