The correct answer is D. Income of consumer should not change.
The law of demand states that, all other things being equal, the quantity demanded of a good or service will decrease as the price of that good or service increases. In other words, there is an inverse relationship between price and quantity demanded.
The assumption that income of consumer should not change is necessary for the law of demand to hold true. If income changes, it will affect the demand for a good or service, even if the price of that good or service does not change. For example, if people’s incomes increase, they will be able to afford to buy more goods and services, including the good or service in question. This will increase the demand for the good or service, even if the price of the good or service does not change.
The other options are not assumptions of the law of demand. Price of the commodity, quantity demanded, and supply are all factors that can affect the demand for a good or service, but they are not assumptions of the law of demand.