Answer
A
Explanation
All of the other options are plots
To the story and a theme is the message that you are trying to get when as a plot is the what happens in the story
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.
Since there is no choice options, there could be lots of different answers like: survivers, travelers, etc. It all depends, can you put the options (if there is any) in the comments of my question? Then i can answer, thanks!