Verified answer

B is correct. Look at the structure of just the clause: "when the food will be ready". The subject, "the food", has to come before the noun in statements. It's possible in Spanish to put the subject of a sentence in different positions, but that's not an option in English. The subject has to come before the verb.

Your sentence isn't correct because the clause--"when will the food be ready"--is structured like a question. The auxiliary verb is placed before the subject, so that's why it's structured like a question. You don't want a question structure in this sentence though--you want a statement structure. In general, if a clause is part of a larger sentence (e.g. this clause is just a part of the complete sentence, "Ask her when..."), then you want to use a statement structure, and not a question structure.

Sometimes a native English speaker might say your sentence if they're speaking a bit lazily or if they misspeak a little. But it's not the correct way to say the sentence and you shouldn't intentionally say it that way.

Subject - Verb - Object (not really an object in this case, but works in the same way)

the food - will be - ready