Aren't both options correct? The restaurant's food and the restaurants' food?
The entire question didn't get reproduced here on the discussion page. I'm pretty sure what it asked was: Fill in the blank: Bia na _ And the two options were bialainne and mbialann. And it only gave you a translation after you picked one or the other. So my argument was that both of the options made grammatical sense, and an English translation to work from was only supplied after you chose one of the options.
I am confused; "na" was not used as the genitive singular article in the previous part (part 1) of this lesson, for example, with "an fhir", "an madra". We were even given the 'hint' that the article is how one can distinguish whether the noun is intended to be genitive singular (i.e., when preceded by 'an') or nominative plural (i.e., if preceded by 'na').
It depends on the gender as well as the number. An can be the nominative singular for either gender, and it can also be the genitive singular for masculine nouns. Na is for feminine nouns in the genitive singular. Na is also the genitive plural for both genders.
These guys may be better at explaining it (scroll down to "In the genitive case"). If you're really into the grammar side of things and like tables, you can look here, but he goes way more in depth, and it might be too much.
I had been trying this as well and got it marked wrong. First I thought the issue was the genitive construction apostrophe + s would be preferred over the "of" genitive here. But then I tried "The food of the restaurant" and it passed. So it seems to be the definite article on food being required. Or is it? Somewhat confused...
You're confused because English is weird.
"The restaurant's food" and "the food of the restaurant" both mean the same thing in English, even though one of them has two definite articles and the other only has one. They both translate into Irish as bia na bialainne, with the single definite article governing the whole phrase, just as it does in "the restaurant's food".