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.
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.