What was the lesson of the first battle of bull run for president abraham lincoln? a. the war could take longer than he had expected. b. the south did not have enough troops. c. the north did not need as many troops as he originally thought. d. the war could end faster than he had expected.

A is the answer.  The first Bull Run was early in the war, and much of the North was convinced it would end quickly-people had even brought out chairs to watch the firefight.  However, their visions were soon shattered.
B is wrong: the South had plenty of troops, and even if it did not, Bull Run was no indication.
C is wrong: Bull Run was a failure for the North, so it would not indicate to Lincoln that the North was overstocked on troops.
Similarly, D is wrong: the war actually would take longer.