The correct answer is: A. estimate cost function.
The fifth step in quantitative analysis of estimating cost function is to estimate cost function. This is because the cost function is the relationship between the quantity produced and the total cost of production. It is important to estimate the cost function in order to determine the optimal level of production.
The other options are incorrect because they are not relevant to the fifth step in quantitative analysis of estimating cost function.
Option B: estimate price function. The price function is the relationship between the quantity demanded and the price of a good or service. It is not relevant to the fifth step in quantitative analysis of estimating cost function because the fifth step is to estimate the cost function, not the price function.
Option C: estimate supply function. The supply function is the relationship between the quantity supplied and the price of a good or service. It is not relevant to the fifth step in quantitative analysis of estimating cost function because the fifth step is to estimate the cost function, not the supply function.
Option D: estimate demand function. The demand function is the relationship between the quantity demanded and the price of a good or service. It is not relevant to the fifth step in quantitative analysis of estimating cost function because the fifth step is to estimate the cost function, not the demand function.