The correct answer is B) The Union blockade kept many from arriving.
During the Civil War, the Union benefitted from a steady stream of new immigrants, some of whom fought in the war. The reason that kept the South from enjoying the same measure of help was that the Union blockade kept many from arriving.
The Union's Army blockade during the Civil War tried to impede the navigation to the Confederated States ports. This way, the Southern States could not receive supplies and could not trade the precious cotton with Europe.
President Abraham Lincoln started the blockade on April 19, 1861. The Union Navy had 450 or 500 navy ships patrolling the East Atlantic coast and the Gulf of Mexico. Only small ships could enter. Immigrants ships couldn't pass through the blockade and they also knew that many job opportunities were in the industries of the North, not in the plantations in the SOuth that used slaves.