You clearly don't have children.Half my conversations right now seem to go: "She eats fish and white bread, but not banana, and he won't touch anything but chicken and rice." It's all in context.
English allows: "He eats orange," "He eats an orange," "He eats the orange," and "He eats some orange." They all have different meanings, but they're all correct. The first specifies the type of food the person habitually ingests. The second tells us the person ate a single orange. The third tells us the person ate a specific single orange. The last tells us the person ate an unspecified quantity of orange.
For me, the first and fourth would both have to be plural - 'He eats oranges' and 'He eats some oranges'. And I would wager that it is likely that way for the course creators as well. But, to the original question, just err on the side of caution and put the article in unless it makes absolutely no sense.
I understand MaireSmith’s question to be about the suitability of “orange” as a mass noun. We’re accustomed to foods like meats as being mass nouns, e.g. “He eats chicken (e.g. because he’s not a vegetarian)”, but foods like fruits as mass nouns are only used in more limited contexts, and discussing what toddlers will eat is one of those contexts: “She eats orange (e.g. if there isn’t much pith on what’s given to her)”. I don’t know how similar the usage of food mass nouns in Irish is to English usage; meats at least seem to be similar, but I don’t know about fruits.
EDIT: I wonder if it has something to do with the size of the food? Larger foods like “pineapple” and “lettuce” seem to be acceptable as mass nouns in more contexts than smaller foods like “raisin” and “pea” are.
(I’m replying here because the comment that I’m actually replying to doesn’t have a Reply link.)
The usage of meats as mass nouns in Irish is certainly similar to English, e.g. Itheann sé sicín could be translated as either “He eats a chicken” or “He eats chicken” depending upon context, but I haven’t seen an example Irish sentence in a reference work with a fruit as a mass noun. The NEID has a sentence in which a fruit mass noun might have been used, but wasn’t:
dip the banana pieces in lemon juice to prevent browning tum na píosaí banana i sú líomóide chun nach n-éireoidh siad donn
This might have been to note that the banana had been sliced into pieces before cooking (to avoid ambiguity with dipping a single unsliced banana), or it might have been because IE English tends to not use fruit as mass nouns (and thus prefers usages like “banana pieces”), or it might have been because Irish tends to not use fruit as mass nouns (and thus prefers usages like píosaí banana); there aren’t enough data to draw a conclusion. Perhaps Knocksedan or other Irish people will comment on the suitability of fruit as mass nouns in IE English.
EDIT: The EID gives
Bia (i dtoradh)
as a translation for “(fruit) flesh”, so perhaps bia i mbanana or bia an bhanana would be an unambiguous way to translate the mass noun “banana” vs. the countable noun “banana”.