The correct answer is: C. The decrease in capital necessary to keep MPL constant when labour increases by one unit.
An isoquant is a curve that shows all the combinations of capital and labor that can produce the same output. The slope of an isoquant measures the marginal rate of technical substitution (MRTS), which is the rate at which labor can be substituted for capital while keeping output constant. The MRTS is always negative, because labor and capital are always complementary inputs. This means that if you want to increase the amount of labor you use, you will need to decrease the amount of capital you use, and vice versa.
The MRTS is also equal to the marginal product of labor (MPL) divided by the marginal product of capital (MPK). This means that the slope of an isoquant tells you how much more labor you need to use to get one more unit of output, given a decrease in capital.
For example, if the MRTS is 2, then you need to use 2 units of labor to get one more unit of output, given a decrease of 1 unit of capital.
The other options are incorrect because:
- Option A is the definition of the marginal rate of substitution of capital for labor (MRSKL).
- Option B is the definition of the marginal product of labor (MPL).
- Option D is the definition of the marginal product of capital (MPK).