What your daughter wrote is fine. 'Candy' can either be singular or plural, it can refer to a specific type of candy or candy in general, and it can be either countable or uncountable. So, 'My favorite food is candy,' is how I would say it in America. In the US, we rarely use 'candies.' Also correct: 'Various types of candy' 'An assortment of candy' 'I like candy of all sorts' 'I bought five pieces of candy, but I didn't eat it all.' 'In my life, I have eaten a lot of candy.' 'I want a piece of candy.'

Matt is right. "My favorite food is candy" is correct. "My favorite food is candies" sounds strange to me (American).

In the sentence "My favorite food is ________", you may fill the blank with either a singular or plural noun. Hence, it is almost impossible to make a mistake. The fact that "candy" can be singular or plural has no relevance. Here are some examples: My favorite food is potato. My favorite food is potatoes. My favorite food is cake. My favorite food is cakes. A singular noun might seem to be better from a grammatical perspective, since "food" is singular. However, plurals work just as well because a plural represents a category (singular!) of food. Any plural you use will be reinterpreted as a singular category. "Candy" and "candies" are both correct, and the reason has nothing to do with being singular or plural. "Candies" is correct because it will be interpreted as meaning not many candies (plural), but rather as the category of food that contains candies (singular).

I think your daughter was correct. "Candy" can be both an uncountable noun and a countable noun. It is commonly an uncountable noun. "She is making candy," "Candy is bad for your teeth," "There is always candy in the supermarket checkout aisle." It's pretty subtle, though. It's one of these things native speakers know without being taught, and have trouble explaining. "I like to eat candy" is correct. "I like to eat candies" is also correct. So it can be countable. Something about the context and the use of the word "food" makes "candies" wrong in that context. "I like to eat fish" is correct. "I like to eat fishes" is incorrect, or at least very strange. "My favorite food is rice" is correct. "My favorite food is rices" is obviously wrong. But "my favorite food is apple" is wrong. It has to be "My favorite food is apples." A US politician was asked what his favorite food was, and got laughed at for saying "My favorite food is hot dog," because as far as I know "hot dog" is never an uncountable noun. It sounded very strange.

Thank you so much for your answers! I'll tell my daughter!