Answer : 3. "Japanese attack on Pearl Harbor"
Explanation :

The primary reason for America's involvement in World War II was:

(C) Japanese attack on Pearl Harbor

The United States entered World War II on December 7, 1941, after the surprise attack by Japan on the U.S. naval base at Pearl Harbor in Hawaii. Prior to the attack, the U.S. had pursued a policy of neutrality, but the bombing of Pearl Harbor prompted the U.S. to declare war on Japan, and subsequently, Germany and Italy declared war on the United States.