Assuming the original sentence is <span><em>As a child, Maria Mitchell studied astronomy with her father William Mitchell, an amateur astronomer</em>, the correct answer here is William Mitchell. An appositive phrase is used to further describe a particular noun next to it, and is usually separated from the rest of the sentence using commas, as is the case here. The appositive phrase <em>an amateur astronomer </em>refers to the father, William Mitchell. </span>
