The third graph. He stops twice, once at the stop sign and again when he reached his destination. on the graph this would be represented with the line at zero which means it can't be the first graph. since it says that he started at a constant speed that means the line doesn't start at zero (since zero indicates no movement and a diagonal line indicates acceleration and he did not accelerate to that speed he started at that speed) and the only other graph that doesn't start at zero is the third graph. This graph follows the journey described as it goes to zero for when he stopped at the stop sign and then accelerates up to the constant speed (the horizontal line) and the decelerates to a stop when he reached work (at the zero).
