To determine the real cost of a unit of food when alternative R is selected from the production possibilities table, we need to analyze the trade-offs between the production of shelter and food at that specific alternative. Step-by-Step Explanation

1. Understanding the Production Possibilities Table: The table shows the maximum combinations of two goods (shelter and food) that can be produced with available resources. The alternatives (P, Q, R, S, T) represent different points on the production possibilities frontier (PPF).

| Alternatives | Units of Shelter | Units of Food | |--------------|------------------|---------------| | P | 5 | 0 | | Q | 4 | 5 | | R | 3 | 9 | | S | 2 | 12 | | T | 0 | 15 |

1. Identifying the Relevant Alternatives:
2. At alternative R, we can produce 3 units of shelter and 9 units of food.
3. To find the real cost of producing one additional unit of food, we need to look at the alternatives immediately before and after R, which are Q and S.
4. Calculating the Trade-off:
5. Moving from alternative R to Q:
   • We decrease shelter from 3 units to 4 units (a decrease of 1 unit of shelter).
   • We increase food from 9 units to 5 units (a decrease of 4 units of food).
6. Moving from alternative R to S:
   • We decrease shelter from 3 units to 2 units (a decrease of 1 unit of shelter).
   • We increase food from 9 units to 12 units (an increase of 3 units of food).
7. Finding the Real Cost:
8. The real cost of producing food is measured in terms of the shelter that must be given up.
9. From R to S, we can see that to produce 3 additional units of food (from 9 to 12), we give up 1 unit of shelter.
10. Therefore, the cost of producing 1 additional unit of food (from 9 to 10) is:
    • 1 unit of shelter / 3 units of food = 1/3 unit of shelter per unit of food.

Conclusion on the Correct Answer Thus, the real cost of producing one unit of food when alternative R is selected is C. one third of a unit of shelter. Why Other Options Are Incorrect

• A. One unit of shelter: This option suggests that producing one unit of food costs one full unit of shelter, which is incorrect based on our calculations.
• B. Three units of shelter: This option implies that producing one unit of food costs three units of shelter, which is not supported by the data.
• D. Six units of shelter: This option is far too high and does not reflect the trade-offs shown in the table.

Revision Summary

• The production possibilities table illustrates trade-offs between two goods.
• The real cost of a good is determined by how much of another good must be sacrificed.
• At alternative R, moving to S shows that 1 unit of shelter is sacrificed for 3 units of food.
• Therefore, the cost of one unit of food is one third of a unit of shelter.