Final answer:
The primary reason for the growing threat of terrorism in the Middle East during the Reagan administration was the support for anti-Soviet Mujahideen in Afghanistan. This support contributed to a prolonged conflict in Afghanistan and the rise of militant groups that eventually became terrorist organizations.
Step-by-step explanation:
The primary reason for the growing threat of terrorism in the Middle East during the Reagan administration was B) Support for anti-Soviet Mujahideen in Afghanistan.
The Reagan administration provided direct and covert aid to the mujahideen, Islamist insurgents who fought against the pro-Soviet Afghan regime. The mujahideen regarded the Afghan government and the Soviet Union as enemies of religion and waged a guerrilla war against them. This support led to a prolonged conflict in Afghanistan and contributed to the rise of militant groups that eventually morphed into terrorist organizations.