The law of demand is illustrated when an increase in the price of a good or service leads to a decrease in the quantity demanded, and a decrease in price leads to an increase in quantity demanded. Among the examples provided, the correct illustration of the law of demand is:
"An increase in the purchases of computer tablets results from lower prices."
This statement reflects the principle of the law of demand, as it shows that lower prices lead to an increase in the quantity of the good (computer tablets) that consumers are willing to purchase.