How to use present to indicate future

Yes, that is a bit of a gray area. There are three ways to express the future in French and English:

1) Present tense: Je vais au cinéma plus tard. -- I'm going to the movies later.
2) Futur proche: Je vais aller au cinéma. -- I'm going to go to the movies.
3) Futur simple: J'irai au cinéma. -- I'll go to the movies.

They are listed in ascending order of "futureness". I think this correspondence should be reflected in grading the test answers as well.

That's not correct, Chris, you can also use the present continuous: "I'm going to the restaurant later".

I agree with Kathleen, both answers should be marked correct. If this were a question where you had to pick a single correct answer, then you would of course choose "I'm going" in preference to "I will go" because it's the closest translation. But since you're asked to give all possible answers, I would expect "I will go" to be included, too. Or, at least, I would be uncertain about it, and rather annoyed to be marked wrong.

I agree with you, Alan, that the grading in the test is, at least, ambiguous and should be changed. On the other hand, though, the present continuous tense in English requires an adverb to connote the future, which is not present in the French original. Strictly speaking, I would say that this is a stylistic possibility to express a future event and not a grammatical one. You would need some context to use it.

Both English and French require context, the French sentence included "plus tard". If you read the question again, you'll see that the present continuous version is marked correct, the only doubt is about the "I will go" version.

That's the "correct answer"?

Edit - I see it's now been changed so it's more accurate, but still doesn't really answer the question. Perhaps someone at Kwiziq could check whether "I will go ..." is really intended to be marked incorrect. Normally when there are questions allowing multiple answers, I think slight nuances in meaning are not considered significant.