Question

The graph below shows the velocity f(t) of a runner during a certain time interval:

Answer 1

First you draw the graph described by the problem to see a better picture of the situation. From the problem the points on the graph are (0,6), (6,8) and (8,0). The first point meant that at time zero the velocity is 6m/s. Then when time is at 8 sec the velocity is zero. The answer to the question is the last choice. 

Answer 2

When graphs are plotted, graphs as in the .jpeg image in attachment are obtained.
First graph is line going through ordered pair (point) (0,6) and pair (6,8).
The second graph is line going through ordered pair (6,8) and pair (8,0).
Intercept of these graphs is point (ordered pair) (6,8).
You see from the .jpeg image that the following is true:
Initial velocity of the runner is 6 meters per second (runner starts with this velocity), for a while he runs, velocity grows, then in the 6th second (time=6), runner starts to slow down and velocity starts to decrease, and runner stops totally at time = 8.
So runner stops after 8 seconds.
The 4th claim (sentence) is correct.