How is a stalling speed not affected by airspeed?
Flying at too low an airspeed will result in a stall, but what "too low" is entirely depends on the situation. For example, if you try a very steep climbing turn, you can stall at an airspeed where you would be fine in level flight.

The answer is an incorrect answer, not because it's factually wrong, but just because it's not general enough. Exceeding the critical angle of attack always causes a stall, and it is the cause of every stall, so it's the best answer to the question. The examiner wants you to understand that the angle of attack is the mechanism that causes stalling, and that it can happen at different speeds.

It's quite common for these (and other multiple choice) exams to have an "incorrect" answer that isn't completely wrong, but isn't as good as the correct answer because it only applies in some conditions or is less general in some other way. In this case, another "incorrect" answer would be "pulling the stick back too far". That's an action that might result in a stall, if you're already at a high angle of attack, but it's not really the cause and isn't always true.