Which characteristic is true about demand curves in an economy?

The characteristic that demand curves are customarily depicted as a straight line for simplicity is accurate because, in economics, a straight-line representation of demand curves allows for easier analysis and understanding of the relationship between price and quantity demanded. While real-world demand curves can take more complex shapes due to varying elasticity and consumer behavior, using a straight line simplifies calculations and graphical interpretations.

In a graphical representation of demand, the straight line illustrates the law of demand: as the price of a good decreases, the quantity demanded typically increases, and vice versa. This linear depiction serves as a foundational concept when teaching economic principles, enabling students to grasp the key elements of demand without becoming overwhelmed by complexities.

In contrast, the other options don't accurately represent the general properties of demand curves. For example, demand curves are not always vertical, as this would imply perfectly inelastic demand, which is not commonplace. Fluctuations with market trends do indeed occur, but describing a demand curve as permanently variable does not reflect the conventional portrayal of demand in economic models. Lastly, stating that demand curves cannot change over time overlooks the reality that shifts in consumer preferences, income levels, and market conditions can cause demand curves to shift, reflecting changing economic dynamics.