Answer = 36.0 mph
As per the question,
Actual speed of the car = 40 mph.
Speedometer has a percent error of 10%.
The error can be negative as well as positive.
If the error is negative, it means speedometer is showing less speed than actual speed= - 10%
10 % of 40 = 4
speed shown by speedometer = 40 - (10% of 40) = 36 mph
If the error is positive, it means speedometer is showing more speed than actual speed = +10%
speed shown by speedometer = 40 + ( 10% of 40) = 44 mph
as per the given options, correct option is 36.0 mph