Your answer would be A. "The idea of placing meat on a bun, however, came from the United States" has a simple structure.
A sentence is defined as a group of words that forms a complete thought. Simple sentences are different from the others because they only have one independent clause and do not contain dependent clauses.
In the example in question, there is one subject (the NP "the idea of placing meat on a bun") and one predicate (the VP "however came from the United States"). This structure stands for one independent clause.
The structure used in each of the sentences below is a simple sentence.
A simple sentence is a form of sentence which has one independent clause and has no dependent clauses. It can stand alone for its meaning as a sentence.
Hence, in the above sentence, it contains one finite verb 'came' which makes it a simple sentence and conveys the meaning of the sentence independently.
n the sentence, "in" is a prepositional phrase, but you have to add the words "place of" along. Otherwise it would not count as a prepositional phrase.
