"They have a lot of food in the fridge."
Translation:Ellos tienen mucha comida en la nevera.
You need to at least keep apart the cases when mucho interacts with a noun and when it interacts with a verb. If it interacts with a noun, it has to agree with that noun in gender and number - "muchos coches", "mucha belleza", and so on - and in those cases it'll be in front of the verb, like you'd place it in English - "many cars", "much beauty".
If mucho influences a verb, it'll often appear behind that verb, and it will not change its form, it'll always be mucho. "Me gusta mucho esa falda" - "I like that skirt a lot".
I rushed my answer and said "muchas comidas". I guess mucha comida is preferable when talking about food in general - but in this example would this muchas comidas sound unusual in most cases and regions (Mx)?
Aside, would it be more natural instead to say "muchas cosas" or "muchas opciones" in a casual conversation involving what to eat from the fridge? Doesn't "comida(s)" seem clunky or obvious if we're already in the kitchen/fridge?
Using comida isn't any more clunky than using "food" in English. Having options in the fridge sounds a bit weird, but "muchas cosas" works as well if you want to be more general.
I can only speak from a European viewpoint, but "muchas comidas" would sound a bit weird in this context - unless you have already prepacked meals in your fridge, that is. You can generally think of a plural comidas as "foods" or "meals" in English.
It doesn't require alimento, but it requires that you use the correct adjective with comida. Comida is a feminine noun, so you need to use the feminine mucha with it. If you use mucho, then comida will be wrong, and Duolingo will look for a fitting masculine noun to match.
You don't, it is optional to include the subject pronoun.
Answers without "ellos" (or "ellas") are in the Correct Solutions list.
If your answer is marked as incorrect and you want to engage the user forum, it is always best to share your full submission so it can be completely checked.