"What food is delicious?"
Translation:무슨 음식이 맛있습니까?
무엇/뭐 is the pronoun what whereas 무슨 is the determiner what. A pronoun can stand in the place of a noun whereas a determiner needs a noun (like the words "the", "a", "an", "this", etc.). Take a look at these examples:
무엇입니까? -> It is what? -> What is it?
무슨 책입니까? -> It is what book? What book is it?
책입니까? -> It is a book? -> Is it a book?
Note the two different whats. 무슨입니까? is ungrammatical because 무슨 can't replace a noun.
There are many answers to this both on the forum and in various websites and videos -(in fact, this is such a commonly asked question I am surprised you haven't stumbled upon an answer yet)- but here are some good explanations to get you started;
은/는 have the same usage, the only difference is 는 is used after a vowel -which you already know.
이/가 have the same usage, the only difference is 가 is used after a vowel -which you also already know.
은/는 are usually used for more general statements: "dogs eat meat" whereas 이/가 are for more specific sentences "The dog eats meat" There are also other nuances around these, so I suggest you look at a couple of the sources above to give yourself a better idea of it.
Hope this helped :)