Answer: C. use 0.8 fewer units of capital.
Step-by-step explanation:
The Marginal Rate of Technical Substitution (MRTS) shows how much you can decrease capital or labor by in order to keep production constant if you increase either capital or labor.
It is calculated by the formula:
= Marginal product of labor / Marginal product of capital
= 4 / 5
= 0.8
The firm should use 0.8 fewer units of capital in order to maintain the same production level.