Answer : (A) They all indicate a constant speed. 
 Explanation :
In this case, four line graphs represent speed in m/s. The x-axis is labeled in seconds (time) and the y-axis is labeled in meters (distance). 
The slope of the distance-time graph gives the slope. It can be seen from the attached figure that, in each case, the speed is constant everywhere. 
Hence, the correct option is (A) " They all indicate a constant speed ".