Which event might explain the increase in the number of Republican representatives in South Carolina between 1865 and 1878?

Explanation:

South Carolina was a Confederate State which means that it was defeated and placed under Union occupation after the Civil War.

The era of Reconstruction then came where the Republican party became really strong in Southern States on account of many Northerners (mostly Republicans) going South to help Southern economies recover.

This coupled with the recently freed enslaved people being Republican, ensured Republican dominance in many areas of the South including South Carolina until the end of Reconstruction in 1877.

letter a is the answer