I looked up the meanings of the two words and amenable means open and responsive to suggestion; easily persuaded or controlled. And docile ready to accept control or instruction; submissive. I really dont know what the answer is but i hope the defintions help
The answer would be B, word choice!
This is a simple sentence because in its entirety it is a single clause. There are no multiple independent clauses connected or any subordinate clauses to make it compound or complex.