If a demand curve is vertical, what does this indicate about consumer behavior?

A vertical demand curve indicates that the quantity demanded remains constant regardless of price changes. This scenario typically occurs for essential goods, where consumers will purchase the same amount no matter how high the price rises, reflecting perfectly inelastic demand. In such cases, consumers do not have alternatives or substitutes that they can turn to, and their need for the product is not dictated by its price. This characteristic of consumer behavior underscores the notion that certain goods, often necessities, are non-negotiable in terms of consumption, regardless of fluctuations in price.

This directly aligns with the correct answer that states demand does not change regardless of price, highlighting the level of insensitivity consumers have toward price variations for these particular goods or services.