Correct Option: B Explanation of the Correct Answer The concept of diminishing marginal returns is a fundamental principle in the theory of production in economics. It refers to the phenomenon where, as you continue to add more of one input (like labor) while keeping other inputs (like capital or land) constant, the additional output (or marginal product) generated from each additional unit of that input will eventually start to decrease. Step-by-Step Breakdown:

1. Understanding Inputs and Outputs:
2. In production, inputs are resources used to produce goods and services. Common inputs include labor, capital, and land.
3. Output is the total quantity of goods or services produced.
4. Marginal Product:
5. The marginal product of an input is the additional output that results from using one more unit of that input, while keeping other inputs constant.
6. For example, if you have a factory and you hire one more worker, the marginal product is how much extra output that worker can produce.
7. Diminishing Marginal Returns:
8. Initially, when you add more of one input (like labor), the marginal product may increase because the additional workers can specialize and work more efficiently.
9. However, after a certain point, adding more workers leads to overcrowding or inefficiencies. Each additional worker contributes less to total output than the previous one. This is the essence of diminishing marginal returns.
10. Graphical Representation:
11. If you were to graph this, you would see that the total output curve rises at an increasing rate initially, then at a decreasing rate, and eventually may plateau. The slope of the curve (which represents marginal product) starts high and then declines.
12. Real-World Example:
13. Consider a farmer who has a fixed amount of land. If he keeps adding more workers to the same piece of land, initially, the output will increase significantly. However, after a certain number of workers, the land becomes crowded, and each additional worker contributes less to the overall output.

Why the Other Options are Incorrect:

• Option A: "Increasing the quantity of all inputs results in a proportional increase in output."
• This statement describes constant returns to scale, not diminishing marginal returns. Diminishing marginal returns specifically refers to the situation where only one input is increased while others are held constant.
• Option C: "The total output increases indefinitely as more inputs are added without any change in efficiency."
• This option suggests that output can increase indefinitely without any loss of efficiency, which contradicts the principle of diminishing marginal returns. In reality, as more inputs are added, the efficiency of each additional input will eventually decline.
• Option D: "The productivity of all inputs remains constant regardless of the quantity used."
• This statement implies that productivity does not change with the quantity of inputs, which is not true in the context of diminishing marginal returns. In fact, productivity does change, particularly when one input is increased while others are held constant.

Summary of Key Points:

• Diminishing marginal returns occur when adding more of one input, while keeping others constant, leads to a decrease in the additional output produced.
• Initially, increasing an input can lead to higher marginal returns, but eventually, the additional output from each new unit of input declines.
• This concept is crucial for understanding production efficiency and resource allocation in economics.

By grasping the concept of diminishing marginal returns, students can better understand how businesses make decisions about resource allocation and production levels.