The correct answer is: C. $\frac{{AR – MR}}{P}$
The degree of monopoly power is a measure of how much a firm can raise prices above marginal cost without losing all of its customers. It is often measured as the Lerner index, which is defined as:
$$L = \frac{{AR – MR}}{P}$$
where $AR$ is average revenue, $MR$ is marginal revenue, and $P$ is price.
A firm with a high degree of monopoly power has a relatively flat demand curve, which means that it can raise prices without losing many customers. A firm with a low degree of monopoly power has a relatively steep demand curve, which means that it cannot raise prices very much without losing many customers.
The Lerner index can be used to compare the monopoly power of different firms in the same industry or to compare the monopoly power of firms in different industries. It can also be used to track changes in monopoly power over time.
The other options are incorrect because they do not measure the degree of monopoly power. Option A, $\frac{P}{e}$, is the price elasticity of demand. Option B, $\frac{{P – MC}}{P}$, is the markup ratio. Option D, $\frac{{MC – P}}{P}$, is the profit margin.