The calculated speed of a four-pole split-phase motor is 1800 RPM. However, if you measure the shaft speed, you will find it to be 1725 RPM. What causes this loss of rotational speed? A. Counter electromagnet force B. Magnetic slip C. Centrifugal mechanism loading D. Power line frequency changes

I think the answer is either A or B possibly

In this scenario the most likely reason is D. power line frequency changes. Many AC electric motors can have their RPM reduced by reducing the frequency of the electricity supply. As this is a multi-phase motor if one of the supply phases is marginally different to the other this will impact on rotational speed. It is also common for the alternating frequency of the current and the voltage to become out of phase with each other. This is a common problem in electricity networks and can cause devices like motors to run less efficiently. This concept is called "power factor" and it is caused by different kinds of loads on the network impacting the current and voltage phase.