The primary causes of the Red Scare after World War II can be attributed to a combination of factors, notably the fear of communism and the geopolitical climate of the Cold War. Among the options you've provided, the most fitting explanation is:
The U.S. government wanted to prevent spying by the Soviet Union during the Cold War.
After World War II, tensions between the United States and the Soviet Union escalated, leading to widespread fears of communist infiltration and espionage within the United States. This paranoia was fueled by real events such as the discovery of spies like Julius and Ethel Rosenberg, as well as the involvement of communist parties in various sectors of American society. The government implemented measures to root out perceived communist influences, which often targeted individuals with left-leaning political beliefs, leading to a broader climate of suspicion and hysteria, commonly referred to as the Red Scare.