Answer: d. employ less capital and more labor.
Explanation: Given the marginal product of labor equals 10 and a marginal product of capital equals 50, since the marginal product of capital is now considerably larger than that of labor, increasing labor should be prioritized at this point, because due to the marginal returns on capital may soon start to diminish especially with the high capital rate. And also, high capital rate of $100, which is considerably higher than the labor wage rate of $20, the firm should employ less capital and focus more on labor