At t = 6, the speed is 5/4
Which is the speed at t = 6?
Time t is meaured in the horizontal axis, so first we need to look at that axis.
We want to find the speed at t = 6, we can see that in that value the position changes linearly, so we only need to find the slope of that line.
Two points on that graph are (0, 10) and (8, 20), then the speed of the particle is:
(20 - 10)/(8- 0) = 5/4
The correct option is the last one.