In the case of a short-run production function, the factor ratio remains

The correct answer is: B. Variable

In the short run, the quantity of at least one factor of production is fixed. This means that the factor ratio, which is the ratio of the quantities of the variable factors to the quantity of the fixed factor, can change. For example, if a firm has a fixed amount of capital, it can increase the amount of labor it uses to produce more output. In this case, the factor ratio would increase.

Option A is incorrect because the factor ratio can change in the short run. Option C is incorrect because the factor ratio is not always constant. Option D is incorrect because the factor ratio can change in the short run.