Which of the following shows why individual demand curve for a good usually slopes downward from left to right?

Analyze the question

The question asks why an individual's demand curve slopes downward (left to right). This means that as price falls, quantity demanded rises.

Explain the concept

The law of diminishing marginal utility explains this. Each extra unit of a good consumed gives the consumer less additional satisfaction than the one before. A rational consumer will only buy more units if the price falls to match the lower satisfaction (marginal utility) from each extra unit. Consumers aim to equate marginal utility with price (MU = P), so:

• Higher quantity → lower MU → the consumer is only willing to pay a lower price
• This gives a downward-sloping demand curve

Evaluate each option

• A. More commodities are offered for sale at lower than higher prices. Wrong. This is about supply and the sellers' behaviour, and it is also not true of supply (suppliers offer more at higher prices).
• B. Marginal utility falls as consumption increases. Correct. This is the reason behind the individual's downward-sloping demand curve.
• C. The higher the price, the higher the quantity offered for sale. Wrong. This is the law of supply, which gives an upward-sloping supply curve.
• D. Prices are usually falling when demand is low. Wrong. This describes a market outcome (a shift in demand), not why an individual's demand curve slopes downward.

Answer: Option B.

Exam tip: If an option talks about sellers or quantity offered, it is about supply, so eliminate it in a demand question. Remember the link: Diminishing marginal utility → downward-sloping demand. Other supporting reasons are the income effect and the substitution effect, but marginal utility is the classic JAMB answer.