One of the factors affecting change in demand for a commodity is the _______?

Step 1: Analyze the question The question asks for a factor that causes a change in demand (a shift of the demand curve), not just a movement along it.

Step 2: Explain the concept In economics, two things are often confused:

• Change in quantity demanded: a movement along the demand curve, caused ONLY by a change in the commodity's own price.
• Change in demand: a shift of the whole demand curve, caused by other (non-price) factors, called the determinants of demand: • Income of consumers • Tastes and fashion • Prices of related goods (substitutes and complements) • Population size • Expectations about future prices

Step 3: Check each option

• A. Supply of the commodity: Supply affects the market price and quantity, but it is not a determinant of demand itself. Wrong.
• B. Availability of substitutes: If substitutes are available and cheap (e.g., Peak milk vs. Cowbell), consumers can switch, so demand for the commodity changes. This is a determinant of demand. Correct.
• C. Price of the commodity: This causes a change in quantity demanded (movement along the curve), not a change in demand. Wrong.
• D. Technical know-how: This affects production and supply, not demand. Wrong.

Step 4: Answer The correct answer is B. Availability of the substitutes.

Exam tip: Remember the phrase "own price = movement, other things = shift." If the question says change in demand, avoid the commodity's own price. If it says change in quantity demanded, own price is the answer. Also, technology and supply-related factors belong to the supply side.