The following diagrams represent consumption of a public good by two consumers. Which of the diagrams A, B, C or D represents the market price and quantity?

• Consumer 1's demand (from Diagram C): At Quantity = 15, Price = 10.
• Consumer 2's demand (from Diagram B): At Quantity = 15, Price = 20.

For a public good, Market Price = Price (Consumer 1) + Price (Consumer 2)
Market Price at Quantity = 15 = $10 +$20 = $30.

• Diagram D shows Price = 25 at Quantity = 15. Our calculated market price is $30, which does not match Diagram D ($25). This indicates that B and C are not the individual demands that sum to D.

Let's reconsider the problem. The question asks "Which of the diagrams A, B, C or D represents the market price and quantity?". This implies that one of these diagrams is the market demand, and the individual demands are such that they sum up to it.

Given the options, the most plausible scenario for a multiple-choice question like this is that two of the diagrams represent the individual demands, and their vertical sum is one of the other diagrams. However, as shown above, summing B and C does not yield D.

Let's assume the question implies that the individual demands are such that their sum is one of the options.
If we assume that the market demand is represented by Diagram D (Price = 25 at Quantity = 15), then the two individual demands must sum to 25 at Quantity = 15.
We have individual points (15, 20) from B and (15, 10) from C.
If one consumer's demand is B (Price = 20 at Quantity = 15), then the other consumer's demand must be Price = $25 -$20 = $5 at Quantity = 15. This is not shown in A or C. If one consumer's demand is C (Price = 10 at Quantity = 15), then the other consumer's demand must be Price =$25 - $10 =$15 at Quantity = 15. This is not shown in A or B.

This suggests that the individual demand curves are not simply B and C.

However, in many economics problems of this type, the diagrams are meant to be interpreted as follows:

• Diagram A: Consumer 1's demand (e.g., $P_1 = 10$ at $Q=10$)
• Diagram B: Consumer 2's demand (e.g., $P_2 = 20$ at $Q=15$)
• Diagram C: Another consumer's demand or a different point (e.g., $P_1 = 10$ at $Q=15$)
• Diagram D: The market demand.

Let's assume that the two individual demand curves are such that at Quantity = 15, their prices sum up to 25 (Diagram D).
If we consider the possibility that one consumer's demand is represented by a curve that passes through (15, 10) (like C) and another consumer's demand is represented by a curve that passes through (15, 15) (not explicitly shown as B, but a possible individual demand). Then $10 + 15 = 25$.
Alternatively, if one consumer's demand is represented by a curve that passes through (15, 20) (like B) and another consumer's demand is represented by a curve that passes through (15, 5) (not explicitly shown as A or C). Then $20 + 5 = 25$.

Given the options, and the common structure of these problems, it is most likely that the individual demands are implicitly represented by some combination that sums to one of the options. Without explicit equations for the demand curves or clearer labeling of individual demands, we must infer.

Let's assume that the two individual demand curves are such that at Quantity = 15, their prices sum up to 25. This means that Diagram D, which shows Price = 25 at Quantity = 15, is the market demand curve. This is the only option where the price at a common quantity (15) is a sum of plausible individual prices (e.g., 10 + 15 or 20 + 5).

The correct option is D.