In demand theory, an increase in the quantity demanded of a good means which of the following has occurred, assuming other factors remain constant?

Introduction / Context:
In microeconomics, it is very important to distinguish between a change in demand and a change in quantity demanded. Many exam questions are designed specifically to test whether you understand this difference. A change in demand refers to a shift of the entire demand curve because of factors such as income, tastes or prices of related goods. A change in quantity demanded, on the other hand, refers to a movement along the existing demand curve due to a change in the price of the good itself.

Given Data / Assumptions:

• The question uses the phrase increase in the quantity demanded.
• We assume that other demand determinants such as income, tastes and prices of related goods remain constant.
• Options mention a shift of the demand curve, a decline in price, an increase in income and all of the above.
• The standard law of demand applies: price and quantity demanded are inversely related, other things equal.

Concept / Approach:
When economists say quantity demanded has increased, they mean that buyers are willing to purchase more units of the good at a particular price because the price has changed, causing a movement along the demand curve. This is different from an increase in demand, which would be shown as a rightward shift of the entire curve. A fall in the price of the good itself, holding all other factors constant, will lead to an increase in quantity demanded. Changes in income or tastes shift the demand curve rather than causing a movement along it, and those would be described as an increase in demand, not an increase in quantity demanded.

Step-by-Step Solution:
Step 1: Focus on the precise wording increase in the quantity demanded, which refers to a movement along the demand curve.Step 2: Recall that such a movement occurs when the price of the good itself changes, under ceteris paribus conditions.Step 3: Identify that a decline in price will move the consumer downward along the demand curve to a higher quantity demanded.Step 4: Note that changes in income or tastes cause shifts in the curve and represent changes in demand, not changes in quantity demanded, so the correct option is the one that mentions a decline in price causing a movement along the curve.

Verification / Alternative check:
A helpful way to verify this is to sketch a simple downward sloping demand curve for a good. Mark an initial price and quantity on the curve. Then lower the price and see where the new equilibrium point lies on the same curve. You will find that the new point lies further to the right, representing a larger quantity demanded at the lower price, but the curve itself has not shifted. This is exactly what is meant by an increase in quantity demanded. By contrast, if consumers suddenly like the product more, the whole curve would shift to the right even at the same price, representing an increase in demand.

Why Other Options Are Wrong:
Option A is wrong because a shift of the demand curve to the right is an increase in demand, not an increase in quantity demanded. Option C is incorrect because an increase in income also causes the demand curve to shift, again representing a change in demand, not a movement along the same curve. Option D, All of the above, is wrong because it mixes both a movement along the curve and shifts of the curve, which is conceptually inconsistent with the specific phrase used in the question. Only option B correctly identifies that a decline in price leads to an increase in quantity demanded along the existing demand curve.

Common Pitfalls:
Many learners use the words demand and quantity demanded loosely in everyday language, but in economics they have precise meanings. A frequent exam mistake is to treat any increase in the amount purchased as an increase in demand even when the change is due to price. To avoid this error, practise reading questions carefully and mentally distinguishing between shifts of the curve (change in demand) and movements along the curve (change in quantity demanded). This will help you answer many related multiple choice questions correctly.

Final Answer:
An increase in the quantity demanded means that the price of the product has declined, causing a movement down along the existing demand curve.