We know according to Lagrange's Mean Value Theorem , we have :
If a function is continuous in [a,b] and differentiable in (a , b) then there is a point x s.t :
f'(x) = [f(b) - f(a)] / [ b - a ]
So considering f'(x) as velocity here and f(x) as displacement here and time as x here , so we have :
Velocity at one point in the entire time interval = Slope
= ( 280 - 0 ) / 8
= 35 m/s
= 35 * 18 / 5
= 126 km / hr
Hence D) is the correct answer according to Lagrange's Mean Value Theorem..