B) Moving with zero acceleration.
Step-by-step explanation:
Acceleration is defined as the rate of change of the velocity per unit time:

In a velocity-time graph, this ratio corresponds to the slope of the curve. In fact:
(change in velocity) corresponds to the increase in the y-axis
(change in time) corresponds to the increase in the x-axis
So, acceleration corresponds to the slope of the curve in a velocity-time graph. For cart C, the slope is zero, therefore the acceleration is also zero.