A as can be seen from the figure, the slope of the image changes within 2 ~ 4 seconds, indicating that the acceleration of the maintenance personnel is changing, and they are doing linear motion with variable acceleration. So, A is wrong.

B, in 0 ~ 4 seconds, maintenance personnel move up, the displacement is positive; In 4 ~ 8 seconds, the maintenance personnel moved down and the displacement was negative, so the displacement in these two periods was different. So, B is wrong.

C, in 0 ~ 2 seconds and 2 ~ 4 seconds, the speed is positive, indicating that the speed direction of maintenance personnel is upward and in the same direction. So, C is wrong.

D, the speed image is a straight line in 2 ~ 6 seconds, and its slope is unchanged, indicating that the acceleration is unchanged. Therefore, d is correct.

So choose d