Answer: C. should use more L and less K to cost minimize.
Step-by-step explanation:
The condition equilibrium means that the MPL/w = MPK/r.
What this implies is that the last dollar spent on capital brings in the equivalent change in the output as the last dollar spent on labour.
So inputting the values, we have,
This implies 4/40 = 40/100
This implies 0.10 < 0.40
So therefore return from capital is much, the firm needs to do more of hiring capital and less of hiring labour, so that the marginal product of labour could be able to increase and using more capital would imply fall in marginal product of capital and a state would be reached where the ratios of MPL/w and MPK/r be the same.
So therefore the correct option is C.