Final answer:
The Soviets underestimated the mujahideen due to their lack of knowledge about the local Afghan terrain, and the significant aid and weapons provided by the United States, which turned the tide against Soviet forces in Afghanistan.
Step-by-step explanation:
The Soviets faced greater resistance from the mujahideen than they expected for several reasons. First, the Soviets were unfamiliar with the Afghan terrain (Option a), which provided a significant strategic disadvantage as the mujahideen utilized their knowledge of the local geography to mount an effective guerrilla war. Second, the United States provided weapons to the mujahideen (Option e), which included surface-to-air missiles that proved to be instrumental in taking out Soviet aircraft. This not only enhanced the mujahideen's fighting capabilities but also shifted the war's balance in their favor. The support the mujahideen received from the US, under the Reagan Doctrine, was aimed at curbing the spread of Communism, and was critical in their ultimate success against Soviet forces.