African americans who fought for the union during the civil war were not freed until after the war. were usually led by african american officers. had been recruited in border states. were paid less than white soldiers.

Correct  Were paid less than white soldiers.

Details:

According to a National Archives article on "Black Soldiers in the U.S. Military during the Civil War" -

In addition to the perils of war faced by all Civil War soldiers, black soldiers faced additional problems stemming from racial prejudice. Racial discrimination was prevalent even in the North, and discriminatory practices permeated the U.S. military. Segregated units were formed with black enlisted men and typically commanded by white officers. ... Black soldiers were initially paid $10 per month from which $3 was automatically deducted for clothing, resulting in a net pay of $7. In contrast, white soldiers received $13 per month from which no clothing allowance was drawn.

In 1864, however, the US Congress corrected the problem of unequal pay, and made the measure they passed retroactive, so that black soldiers would receive retroactive wages to compensate for that previous inequality.  

The answer is option c.