Well, normally an assumption is unstated. But someone can explicitly call out an assumption in the course of an argument. For instance, I might say "If we believed X were true, then we would likely expect Y to happen" or "Y will happen if, as I suspect, we don't see a decrease in X." The author might also call out an assumption made by an opposing party. So don't automatically rules out a role answer just because it has the word "assumption." If the author presents something as true, it's a premise. If the author states an opinion as a result of a premise, that's a conclusion. But if the author states something that they THINK MIGHT be true, or that would NEED TO BE TRUE to reach a conclusion, that might be described as an assumption.