The correct answer is: B. equal to unity.
The elasticity of demand is a measure of how responsive consumers are to changes in price. When the elasticity of demand is equal to unity, it means that a 1% increase in price will lead to a 1% decrease in demand. In this case, the marginal revenue will also be equal to unity. This is because the marginal revenue is the additional revenue that a firm earns from selling an additional unit of output. When the elasticity of demand is equal to unity, the marginal revenue is equal to the average revenue.
The other options are incorrect because they do not reflect the relationship between the elasticity of demand and the marginal revenue. When the elasticity of demand is more than unity, the marginal revenue will be less than unity. This is because the firm will earn less revenue from selling an additional unit of output when consumers are very responsive to changes in price. When the elasticity of demand is less than unity, the marginal revenue will be more than unity. This is because the firm will earn more revenue from selling an additional unit of output when consumers are not very responsive to changes in price.