Answer:
C.metaphor.
Explanation:
Metaphor is a literary device which is used by writers or speakers to equate two things while explaining a particular idea. It a part of figurative speech that makes a comparison of ideas to produce one particular idea.
Here, when the author says "[Cora] Martin is part of a small army of young volunteers," she is using a metaphor. By saying that Cora is part of a small army, she means to say that Cora is part of an organisation/group of people.
Some few examples of metaphor are 'This is just the tip of the iceberg', 'Love is in the air', 'She has a fist of iron' etc.
