The correct answer is C) It helps consumers tell producers when prices are too high.
The law of demand is an economic ratio that relates the number of products demanded by consumers and the price that the consumers are able to pay for that quantity. According to this law, consumers are rational actors and, therefore, they prefer to pay a lower price for a larger quantity of goods and, on the contrary, not many people would pay a high price for a small quantity.
Take for example chocolate, if a chocolate costs around 10 cents, then many people would be able to pay for it due to its low cost, but on the contrary, if it costs 50 dollars, then almost nobody would buy it and the quantity demanded would fall to almost zero.
In the graphic, the price is on the ordinate axis and the quantity of goods is represented in the axis of the abscissas. For that reason, the demand curve has a negative slope. The supply curve has a positive slope and at the point in which both intersect is the equilibrium price.
