Question

What took place that quite possibly saved Grant from defeat at Shiloh?

Confederate troops retreated suddenly due to heavy losses.
Union soldiers fought very well.
Buell arrived with reinforcements after the first day of battle.
General Prentiss was able to hold on to his position in the Hornet's Nest.
Reinforcements arrived from General Sherman.

Answer 1

General Prentiss was able to hold on to his position in the Hornet's Nest.

Explanation:

I did a test and got this one right.

Answer 2

General Prentiss was able to hold on to his position in the Hornet's Nest. Together with General William H.L. Wallace, they were able to reform and hold the lines against the Confederate attack. This enabled reinforcements under Buell to turn back the Confederate attack. This ended the efforts of the South to prevent the Union advance to the Mississippi.