The correct answer is: Electric clock.
An electric clock is a device that uses an electric motor to power a gear train that moves the hands of the clock. The motor is typically powered by a direct current (DC) power source, such as a battery or a power adapter. The frequency of the AC power supply does not affect the operation of the clock, as long as the DC power source is providing the correct voltage.
A mixer grinder is a kitchen appliance that is used to mix, grind, and chop food. It typically has two or more blades that are powered by an electric motor. The frequency of the AC power supply can affect the speed of the motor, and therefore the speed of the blades. If the frequency of the power supply is too low, the blades may not turn fast enough to effectively mix or grind food. If the frequency of the power supply is too high, the blades may turn too fast and cause the food to splatter.
A ceiling fan is an appliance that is used to circulate air in a room. It typically has two or three blades that are powered by an electric motor. The frequency of the AC power supply can affect the speed of the motor, and therefore the speed of the blades. If the frequency of the power supply is too low, the blades may not turn fast enough to effectively circulate air. If the frequency of the power supply is too high, the blades may turn too fast and cause the fan to make a lot of noise.
A room heater is an appliance that is used to heat a room. It typically has a heating element that is powered by an electric current. The frequency of the AC power supply does not affect the operation of the heater, as long as the heating element is receiving the correct voltage.
In conclusion, the electric clock is the appliance that will be least affected due to a change in the supply voltage frequency.