U-boats were used primarily to
-gather information about enemy movements by tracking warships.
-protect the german coast from attack by allied ships and submarines.
-resupply troops with guns, ammunition, and food via sea rather than land.
-target allied shipments on passenger ships, warships, and merchant ships.

The correct answer is D. Target allied shipments on passenger ships, warships, and merchant ships

Explanation:

U-boats, also called Underseeboats were German Submarines used to target and sink passenger ships, warship or merchant ships during World Wars. During the First World War from 1914 to 1918, Germany started the implementation of U-boats against warships of the Allied but then extend it to merchant ships; the allied was the coalition of countries against Austria-Hungary, Germany, and Bulgary composed by France, the U.K., Japan, Italy, the U.S., and Russia. In the case of the Second World War from 1939 to 1945,  Germany continued using the U-boats to target and later sink any kind of ships of the allies that were United States, China, the United Kingdom, and the Soviet Union. Thus, U-boats (Germany submarines) were used to target (and later sink) shipments from the allies including warships, merchant ships, and passenger ships.

c. as a force against soviet invasion of afghanistan