The correct response is:
A. A decline in white volunteers and the continuous rise in the need for soldiers of the Union Army led to a change in policy.
This reflects the reality of the Civil War situation, as the Union Army faced shortages of soldiers and turned to enlist African Americans as a solution. The Second Confiscation and Militia Act (1862) and the Emancipation Proclamation (1863) were significant steps in allowing African Americans to serve in the military.