On the highway, the car use 1 gallon per 36 miles
In the city, it uses 1 gallon per 20 miles
No. of gallons used to travel 4 miles on the highway = 4/30 = 0.133
No. of gallons used to travel 4 miles in the city = 4/20 = 0.2
No. of gallons used to travel 8 miles on the highway = 8/30 = 0.267
The % difference from gallons used to travel 8 miles on the highway =
$$\frac{0.333-0.267}{0.267}\cdot\frac{100}{1}$$
$$=\frac{0.066}{0.267}\cdot\frac{100}{1}$$
$$=24.7\approx25\%$$
Answer = option D