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.