The best way to describe the Soviet-Afghan is by calling it a failure, due to the Soviets being forced to withdrawal. One main reason they lost is because they didn’t have a good counter to guerrilla warfare, and continued loosing battles because of it. There were also various other reasons (clothing, no initiative in commanders, etc).
Here are why the other answers are incorrect:
1. The Soviets were not actually able to defeat the afghan insurgents (Mujahideen), and were forced to leave (as answer 3 says).
2. The insurgents and Soviets suffered numerous casualties, but it is correct in the sense of calling the invasion a failure.
4. I don’t believe any reforms were introduced as a direct result of the Soviet-Afghan war.
