The correct answer is D. Cannot be determined with the information given.
The marginal product of labor (MPL) is the additional output produced by an additional unit of labor. It is calculated by taking the derivative of the production function with respect to labor. In this case, the production function is $q = 4L + K$. Taking the derivative with respect to labor gives us $MPL = 4$. However, this is only the marginal product of labor when capital is held constant. If capital is allowed to vary, then the marginal product of labor will be different. To calculate the marginal product of labor when capital is allowed to vary, we would need to know the relationship between capital and output. This information is not given in the question, so we cannot determine the marginal product of labor.
Option A is incorrect because it includes the term $k$, which is the amount of capital. However, we do not know the relationship between capital and output, so we cannot include $k$ in the formula for the marginal product of labor.
Option B is incorrect because it does not include the term $k$. However, we know that the marginal product of labor is not equal to 4 when capital is allowed to vary.
Option C is incorrect because it includes the term $4k$. However, we do not know the relationship between capital and output, so we cannot include $4k$ in the formula for the marginal product of labor.