When the distance an object travels is directly proportional to the length of time, it is said to travel with

The correct option is B. constant speed

When distance traveled is proportional to time means graph between s-t is a straight line and the slope of the s-t graph gives the speed of the object which is constant.

Or

$\text{Speed}=\frac{\text{Distance travelled}}{\text{Time}}$

If speed is constant then, $\text{Distance travelled}\propto \text{Time}$.

So this means body is moving with constant speed.