The correct answer is: Prof. Benham.
Prof. Benham is the only one of the four professors who defines demand as the quantity demanded which is purchased at a fixed price. Prof. Penson defines demand as the willingness and ability to purchase a good or service, Prof. J. S. Mill defines demand as the desire for a good or service backed by the ability to pay, and Prof. Mayors defines demand as the quantity of a good or service that consumers are willing and able to purchase at a given price.
Prof. Benham’s definition of demand is the most common definition used in economics. It is based on the idea that demand is a function of price, meaning that the quantity demanded of a good or service will change as the price of that good or service changes. This is in contrast to the other three definitions, which are based on the idea that demand is a function of other factors, such as income, taste, and expectations.
Prof. Benham’s definition of demand is the most useful definition for economic analysis because it is the most precise. It allows economists to make predictions about how consumers will respond to changes in price. The other three definitions are less precise and are not as useful for economic analysis.