The answer is " d. The Union won a decisive victory over the Confederacy. "
Here's an explanation of what happened:
The commander of the Confederates, A.S. Johnston was killed, and tactical command passed to the planner of the battle plan, Gal. Beauregard who evaluated that Grant (one of the generals of the union), would not be reinforced at night and that his own troops had already disorganized considerably along the advance. So he chose to resume the attacks only the next day. It was a serious mistake, for at dusk Lew Wallace and Buell (who were generals of the union), arrived on the battlefield with 27,000 new soldiers. Grant summed up the day with a heavy offensive, sweeping the Confederates from the positions he'd missed the day before.
The Union victory was an important step towards acquiring dominance of the Mississippi Valley, a waterway of enormous strategic importance. Its seizure by the Union would divide the Confederacy into two parts, stripping the Confederate armies of important recruitment and agricultural production areas vital to the war effort.