Given that,(45−x)×√n ⇒(45−x)=k×√n Where x is the speed of the train, k is the proportionality constant and n is the number of wagon wheels attached. Now given that when n=9,x=30 Thus,15=k×3 Thus, k=5 Thus,45−x=5√n Thus, when n=81,x=0 Thus, for the train to move the maximum number of wagon wheels = 80 Hence, option C is the correct answer.