"She takes care of the food."

Because the infinitive verb "ocuparse" means "to take care of." It may help to think of it as a different verb than ocupar.

Well spoted!

why did it present me with a word I have not learned "cuida' instead of accepting "ocupa"?

Maybe "Ella cuida la comida", but the "a" should not be used in this case (since it's only needed when the object is a person).

I don't think "cuidar" is the appropriate verb. "cuidar de" means "to take care of" in the sense of "tending to the needs of". For example...

"Yo no podía cuidar de mi hijo" = "I could not take care of my son".

"to take care of" has a broad meaning in English, so it could be translated by one of several Spanish verbs, depending on the context. It could mean she is in charge of organizing the food, or that she is cooking the food, or that she is making sure a hungry bear doesn't eat the food, ...

I agree that the multitude of uses of "se" are challenging to learn. Me,te,nos,les are a bit less confusing limiting the challenges to area 1 below. Here is a good summary of the uses of "se". It takes a lot of practicing and exposure to make this intuitive. I certainly need much practice in this area. If you know of more, please post.

http://spanish.about.com/od/pronouns/a/introduction_se.htm

1. Reflexive Pronoun a. To reflect the action back on the subject b. To indicate an emotional response c. To add emphasis d. To change the meaning or nuance of the verb http://www.spanishdict.com/topics/show/85
2. Passive voice
3. As substitute for le or les
4. Impersonal se

I read "Ella ocuparse de la comida" as "She to occupy herself with the food" or "She to take care herself of the food", which don't make sense in English, and I've confirmed with a native speaker that it's wrong in Spanish. Ocuparse is not the same as se ocupa (occupies herself), but se ocupar (to occupy herself).

Ocupar - to occupy. Ellos ocupan el edificio (They occupy the building). Yo ocupo, tú ocupas, él ocupa, etc.

Looking around, "cuidar" and "ocuparse de" are considered synonyms, so I'd think your answer should work.

However, synonyms usually have subtle differences. It'd be nice to hear comments from a native Spanish speaker.

For various reasons: First, the verb has to match the subject. We have a third person singular here, so it has to be ocupa not ocupar, since the latter is the infinitive form (to take care). So what you wrote basically translates to she (to) take care of the food. Second, pronouns can only be attached to either the imperative form (e.g.: damelo - give it to me) or the infinive form (e.g. darlo - to give it). So while ocuparse isn't wrong per se, it's just not conjugated correctly. I don't know what your native language is, but I've experienced that many native English speakers find conjugating verbs quite confusing because in English all verbs have only one form for every tense. The only exception is the third-person-singular-s in the simple present. Most other languages - especially slavic and romance languages - require a specific ending which directly refers to the subject. I hope this doesn't confuse. Explaining grammar through the internet is extremely hard because I can't really tell how much you know about the language already. So ask away!