CONCEPT: Speed: The rate of change of distance is called speed. Velocity: The rate of change of displacement is called velocity. ​Velocity (V) is given by: V = dx/dt Where x is displacement and t is the time taken Acceleration (a): The rate of change in velocity is called acceleration. a = dV/dt CALCULATION: Given that: x = at2 + bt + c Velocity (V) = dx/dt = d (at2 + bt + c)/dt = 2at + b At t= 0 (for initial velocity) initial velocity (u) = 2a × 0 + b = b Initial displacement (x) = at2 + bt + c = a × 02 + b × 0 + c = c Acceleration (a) = dV/dt = d (2at + b)/dt = 2a At any time t, velocity = V = final velocity = 2at + b So option 4 is correct.