The correct answer is $\boxed{\left[ {1 – \frac{1}{e}} \right]}$.
Marginal revenue (MR) is the additional revenue that a firm earns from selling an additional unit of output. It is calculated by taking the derivative of total revenue (TR) with respect to output.
Total revenue is equal to price times quantity sold, so MR is equal to price times the derivative of quantity sold with respect to price. The derivative of quantity sold with respect to price is negative, because as price increases, quantity demanded decreases.
The elasticity of demand is a measure of how responsive quantity demanded is to changes in price. It is calculated by taking the percentage change in quantity demanded divided by the percentage change in price.
A less than perfectly elastic demand curve is one where the elasticity of demand is less than 1. This means that quantity demanded is not very responsive to changes in price.
When the elasticity of demand is less than 1, MR is less than price. This is because the decrease in revenue from selling fewer units outweighs the increase in revenue from charging a higher price.
The formula for MR is:
$$MR = P \left[ 1 – \frac{1}{e} \right]$$
where $P$ is price and $e$ is the elasticity of demand.
Therefore, the correct answer is $\boxed{\left[ {1 – \frac{1}{e}} \right]}$.