The correct answer is A) remaining loyal to Britain.
The interrupter in the sentence “The Middletons, remaining loyal to Britain, become unpopular with their neighbors”, is remaining loyal to Britain.
An interrupter in grammar is any phrase or word that interrupts or considerably breaks the flaw of a sentence. So it is important to punctuate the interrupter in a correct form. This is to separate it from the main idea of the sentence. This is done with the use of commas at the beginning of the word or the phrase and at the end of it.
