Answer:
Right choice:
The war drained resources needed in the Soviet homeland.
Step-by-step explanation:
The Soviet Army had to fight a long guerrilla war in Afghanistan for which their soldiers and command were not well prepared in terms of training and military strategic thinking, nor in terms of economic strength. The invasion took place in 1979. By then, the Soviet economy was already stagnated, it lagged behind the West technologically and the command economy was uncapable of generating wealth efficiently. The cost of fighting this war with its own army - Afghanistan was the Soviet Vietnam -, the financial burden of supplying allies in the proxy wars of the Third World, and the huge cost of the nuclear race with the USA, drained too many resources of a stagnant economy.