A, because $1.68 per mile is your rate, and since you don't know how many miles you will be going, you represent it with x. The initial fee of $2.15 is added to $1.68x so you can get your total cost. If you went 8 miles, your equation would be $1.68(8)+$2.15, however since there isnt a specific number of miles, we will use x.
