By how much can driving smoothly reduce fuel consumption?

Answer: C — 15%

Driving smoothly, as opposed to aggressive driving, can reduce fuel consumption by up to 15%. This is because aggressive driving practices such as rapid acceleration and hard braking can use excess amounts of fuel. Option A is incorrect because it is too low of an estimate. Option B is incorrect because it is also too low of an estimate. Option D is incorrect because it is too high of an estimate. The most accurate answer is option C, 15%.