An object on a number line moved from x = 15 cm to x = 165 cm and then moved back to x = 25 cm, all in a time of 100 seconds.

What was the average speed of the object?

v_avg = 2.9 cm/s

Explanation:

The average velocity of the object is the sum of the distance of all its trajectories divided the time:

x_all is the total distance traveled by the object. In this case you have that the object traveled in the first trajectory 165cm-15cm = 150cm, and in the second one, 165cm - 25cm = 140cm

Then, x_all = 150cm + 140cm = 290cm

The average velocity is, for t = 100s

hence, the average velocity of the object in the total trajectory traveled is 2.9 cm/s

whichever one it is, it's definitely not one that you included on the list you provided.