What factors determine the quantities demanded of a good or service?

The correct choice focuses on the "Determinants of Demand," which are fundamental factors that influence the quantity of a good or service that consumers are willing and able to purchase at various price levels. These determinants include several key elements such as consumer preferences, income levels, the prices of related goods (substitutes and complements), consumer expectations, and the number of buyers in the market.

Understanding the determinants of demand is crucial because they directly affect how demand shifts. For example, if consumers' incomes rise, they may demand more luxury goods, shifting the demand curve to the right. Similarly, if the price of a substitute good increases, consumers may demand more of the original good, again shifting the demand curve.

In contrast, other options do not encapsulate the broad and fundamental factors influencing demand as comprehensively. While elasticity and market competition play a role in understanding how price changes can affect demand, they are not the primary factors that determine the overall quantities demanded. Consumer trends are important but more localized, and they do not encompass the entire framework of demand determinants. Supply constraints relate more to the availability of goods rather than consumer demand itself.