Is it mandatory to turn off the engine when refueling?
A)No.
B)Yes, but it is not necessary if it is the gas station employee who fills the tank.
C)Yes, always.Correct
# Explanation
**Yes, it is always mandatory to turn off the engine when refueling.** This measure is essential for safety reasons, since a running engine generates heat and can produce sparks that ignite the flammable vapors of the fuel. **There are no exceptions to this rule, not even when the gas station staff performs the refueling.**