The correct option is D No error
In this sentence, there is one subject: Bread and butter. This term is generally considered as one item, so it's a singular noun.
We need to choose the option that mentions the error, so the options that do not contain the error, are wrong.
Option A is wrong because the term 'bread and butter' has no error.
Option B: Similarly, there isn't any error in 'is'. The subject is singular, so the verb will also be singular.
Option C is wrong because this part, too, has no error.
Option D is the correct answer because the given sentence has no error, it's grammatically correct.