In the theory of demand, which curve represents the combined demand of all individual consumers in the market for a good at different price levels?

Introduction / Context:
Demand analysis distinguishes between the behaviour of a single consumer and the behaviour of the market as a whole. While individual demand curves show how one consumer responds to price changes, examinations frequently ask what we call the curve that aggregates the behaviour of all consumers. Recognising this term is essential for understanding market equilibrium in microeconomics.

Given Data / Assumptions:

• We have many individual consumers for a good in a market.
• Each consumer has an individual demand schedule and a corresponding demand curve.
• We are interested in the total quantity demanded by all consumers at each price.
• The question asks for the name of the curve that represents this combined demand.

Concept / Approach:
For each possible price of a good, we can add up the quantities demanded by all individual consumers to obtain the total quantity demanded in the market. Plotting these total quantities against the corresponding prices gives the market demand curve. This curve is the horizontal summation of all individual demand curves. It is distinct from indifference curves, which represent preferences of a single consumer, and from abstract curves like diminishing utility curves.

Step-by-Step Solution:
1. For a given price, list the quantity demanded by each individual consumer.2. Add these quantities to obtain the total quantity demanded at that price.3. Repeat the process for different price levels to construct a schedule of total quantity demanded.4. Plot the price on the vertical axis and the total quantity demanded on the horizontal axis.5. The curve that joins these points is called the market demand curve, because it shows the aggregate demand of all consumers at each price.

Verification / Alternative check:
Imagine a simple market with two consumers. At a price of 10 units of currency, consumer A demands 3 units and consumer B demands 2 units, so total market demand is 5 units. At a price of 8, A demands 4 and B demands 3, so total demand is 7. Plotting these totals against price yields a curve that reflects the combined behaviour of both consumers. Extending this idea to many consumers gives the market demand curve. This confirms that the curve described in the question is the market demand curve.

Why Other Options Are Wrong:
Option A: Monotonic demand curve is not a standard term. While demand curves are usually downward sloping, the accepted name for the aggregate curve is market demand curve.
Option B: An indifference curve shows different combinations of two goods that provide equal satisfaction to a single consumer. It does not show quantity demanded at different prices.
Option D: Diminishing utility curve is more of a conceptual phrase and not the correct term used for the aggregate demand curve.
Option E: Individual demand curve refers to one consumer only, not to all consumers combined.

Common Pitfalls:
One common mistake is to confuse individual and market demand curves. Remember that the market demand curve is obtained by horizontal summation of all individual demand curves, not by averaging prices or vertically adding quantities. Another pitfall lies in mixing up demand curves with indifference curves, which live in a different diagram (goods space rather than price quantity space). Keeping track of what is on each axis helps avoid this confusion.

Final Answer:
The curve that represents the combined demand of all consumers in the market at different prices is called the market demand curve.