The question stem tells us the following:

Cindy drives roughly 290 miles. But that distance is rounded to the nearest 10 gallons. So Cindy could have driven anywhere between 285 - 294 miles. 285 rounds to 290. 294 rounds to 290 as well. 285 is the minimum and 294 is the maximum of this range.

Cindy uses roughly 12 gallons. But that is rounded to the nearest gallon meaning that Cindy could have used anywhere between 11.5 - 12.4 gallons.

You are asked to find the range of miles per gallon. To do this, you need to find the minimum and maximum.

Minimum is 285 miles/12.4 gallons. You divide the minimum distance by the maximum gallon consumption because you want the answer to the least in value.

Your maximum is 294 miles/11.5 gallons. This time you divide the maximum distance by the minimum gallon consumption because you want the answer to be the greatest in value.

The answer that most closely resembles this is D. Hope that helps!

when we say rounded that means the digit must lie between x+.49 0r x-0.49

so in our case here for simplicity we take x+5 and x-5

miles covered can range from 285 to 295
gallons used can range from 11.5 to 12.5

worst millage is going to be when distance covered is low and gallons used is high = 285/12.5
best millage is going to be when distance covered is high and gallons used are low = 295/11.5

285/12.5<=millage<= 295/11.5