Final answer:
The correct answer is b. Siege of Vicksburg, which resulted in the Union gaining control of the Mississippi River under General Ulysses S. Grant, marking a pivotal moment in the Civil War by splitting the Confederacy in two.
Step-by-step explanation:
The attack by Union general Ulysses S. Grant that gave the North control of the Mississippi River was the Siege of Vicksburg. Following several failed attempts to take the city, Grant's forces encircled Vicksburg and began a siege in May 1863. After enduring 47 days of encirclement, with supplies nearly exhausted and no meaningful reinforcements available, Confederate forces in Vicksburg finally surrendered on July 4, 1863. This victory was pivotal as it gave the Union complete control over the Mississippi River, which was vital for the North's strategic goal of splitting the Confederacy in two and crippling its ability to maintain the war effort.
The Union forces, commanded by Grant, were successful in weakening the Confederacy's stronghold in the West by gaining control of key points along the Mississippi River. Vicksburg's fall, combined with the Union's defeat of the Confederacy at Gettysburg, greatly turned the tide in favor of the Union. The victory at Vicksburg was one of General Grant's most significant accomplishments during the Civil War and played a crucial role in the eventual Union triumph.