The diagram shows an electric motor pulling a truck of mass m along a slope - Edexcel - A-Level Physics - Question 6 - 2023 - Paper 1

To determine the efficiency of the motor, we can utilize the formula for efficiency, which is given by the ratio of useful output power to the input power.

The useful work done (output energy) by the motor can be calculated as the gravitational potential energy gained by the truck, which is given by:

$W = mgh$

The input energy supplied to the motor during the time t can be calculated as:

$P_{in} = VIt$

Thus, the efficiency 'η' can be expressed as:

$\eta = \frac{\text{Useful Output Energy}}{\text{Input Energy}} = \frac{mgh}{VIt}$

This matches option D. Therefore, the correct expression for the efficiency of the motor is:

D) ( \frac{mgh}{VIt} )