Which of the following alternatives is grammatically correct?

The correct option is B: "Bread and milk is a wholesome food."

Grammatical Rule: While "bread and milk" are two separate items, they are often treated as a singular compound idea or a single unit when referring to a meal. In such cases, the singular verb "is" is used.

Contextual Application: Because "bread and milk" is being described as a singular "wholesome food" (a unit), the singular verb "is" correctly matches the singular noun phrase that follows.