Point C.
Explanation
[tex]\textbf{Velocity} = \dfrac{\text{Change in }\textbf{displacement}}{\text{Change in Time}} = \dfrac{\Delta {\bf x}}{\Delta t}[/tex].
[tex]\dfrac{\Delta x}{\Delta t}[/tex] is the same as the gradient of the tangent to the position-times graph at the point in question.
Draw a tangent line to the curve at each point. Measure the gradient of each tangent line. The gradient of the tangent line to the curve is smallest at point C among all four points. Therefore, the car's velocity is smallest at point C.
