Flash floods can cause vehicles to float and fill with water, trapping and drowning people. while especially dangerous at night and in deep water, even inches of water can float some small cars.
a. two
b. four
c. six
d. eight

I think the answer is: 6 inches

On average, the distance between Car's engine to the ground is around 8-10 inches.. But for small cars, the distance between car's engine to the ground could be as low as 5 inches - 6 inches from the ground.

When the flood reach the car's engine level, the water will got into the engine and disrupt it function, causing the car became unable to work properly