By adding "south", the speed had both magnitude AND direction, becoming a vector, not a scalar which generic "speed" is. So the second option can be eliminated. Let's analyze the other part(s). A velocity is a distance/time. Do we have that? Yes! -> miles/hour. So, velocity is an option that theoretically fits this question. Buttt... It gives us a time, and acceleration is a change in a velocity (speed, with a direction)/time. So because it says "after 10 minutes", we consider the big picture to be talking about the change in velocity during that time span, which is simplified as "the acceleration" over that time. :)
