(a) is
"Bread and butter" is treated as a single unit (one item), so it takes a singular verb — is.
When two things are habitually combined and referred to as a single concept (e.g., "bread and butter," "rice and curry"), the verb is singular. This is a standard Subject-Verb Agreement rule tested in CBSE grammar. Options (b) "am" is for first person singular, (c) "are" is plural, and (d) "has been" is grammatically possible but contextually unnatural here — "is" is the correct and expected answer.