Final answer:
The market demand curve shows the total quantity demanded of a good by all consumers at various prices, representing the combined preferences of consumers for that good across different price points. Option d. is correct.
Step-by-step explanation:
The proper definition of a market demand curve is: A market demand curve shows the total quantity demanded of a good per period by all consumers at various prices. This definition emphasizes that the market demand curve reports the collective demand for a good or service from all consumers in the market, not just a single individual or those with the same income. Moreover, it specifies that this relationship is shown over various prices. It aligns with the law of demand, which indicates that generally, as the price of a good increases, the quantity demanded decreases, and vice versa.
A demand schedule is typically used as a table format to show the quantity demanded at different prices, while a demand curve represents this relationship graphically, with price on the vertical axis and quantity demanded on the horizontal axis.