Final answer:
The law of demand indicates an inverse relationship between the price of a commodity and the quantity demanded, where a higher price reduces demand, and a lower price increases it.
Step-by-step explanation:
The law of demand highlights the relationship between the price of a commodity and the quantity demanded. Specifically, it explains that there is an inverse relationship where a higher price results in a lower quantity demanded, and conversely, a lower price leads to a higher quantity demanded. This relationship holds true while other variables remain constant. Therefore, the correct answer to the student's question is A. Price and quantity of a commodity.