Answer:
Broadest: The use of metaphor by American poets.
Middle: The use of metaphor by Emily Dickinson
Narrowest: The use of metaphor in Emily Dickinson's "The Railway Train"
Step-by-step explanation:
Broadest is the all American poets because there are many.
Middle is the Emily Dickinson because she is only one poet.
Narrowest is the use of metaphor in Emily Dickinson's "The Railway Train" because it is one poem by one poet.