Answer:
x represents Annette speed in miles per minute. Hence, option b) is correct. 
Explanation:
Given that Annette rode her bike 6.8 miles at a constant rate for 90 minutes. Also to find how fast she traveled, she solved the equation 
 . Now, we have to tell here what x represents.
. Now, we have to tell here what x represents.
As, 

Let speed of bike is x miles/min and given distance is 6.8 miles and at a constant rate for 90 minutes.
∴ 

⇒ 

which is the same equation as given.
Here x represents Annette speed in miles per minute. Hence, option b) is correct.