A His capture of Vicksburg prevented shipments of Confederate supplies from reaching the West.
William T. Sherman's siege and eventual capture of Vicksburg led to the severing of the western half of the confederate states from the east, leading to the South having problems moving supplies and man across, and effectively cut off all trade, weakening the CSA.
B His march destroyed Georgia and demonstrated the military superiority of the Union troops.
Sherman's famous "March to the Sea" was directed through the state Georgia, in which the soldiers destroyed many infrastructures, and burned crops. They also freed slaves in the surrounding area, which swelled their ranks.
D His capture of Atlanta facilitated the reelection of Lincoln as president.
Many Northerners were complaining that Lincoln was not doing much to help the war (as the North progressed slowly), and looked to a general that Lincoln had laid off, George McClellan, who headed the Democratic Party and promised a speedy treaty with the South with favorable terms for both sides. However, victory by Lincoln's generals meant that the people had faith in him again, and so they reelected him.
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It is NOT C, because Sherman's march to the sea was through Georgia, and in no way did it progress near the Gulf of Mexico (meaning that he cut through Texas).
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