There is no article or answer choices so how are people gonna answer this
The essential appositive in the sentence "astronomer maia mitchell discovered a comet in 1847 and went on to become the first professional female astronomer in the united states," is Maia Mitchel. The last option is correct.
An appositive consists on a noun or noun phrase that renames another noun right beside it. The appositive can be a short or long combination of words.
In this particular case, the words Maia Mitchell which is a proper noun is renaming the noun astronomer, which is right before it and constitutes the subject of the sentence.
