B) Moving with zero acceleration.
Explanation:
Acceleration is defined as the rate of change of the velocity per unit time:
[tex]a=\frac{\Delta v}{\Delta t}[/tex]
In a velocity-time graph, this ratio corresponds to the slope of the curve. In fact:
[tex]\Delta v[/tex] (change in velocity) corresponds to the increase in the y-axis
[tex]\Delta t[/tex] (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.