Answer: d. The market risk premium declines.
Explanation:
The Weighted Average Cost of Capital (WACC) takes into account the cost of equity and debt which means that if either of these costs increase, the WACC will increase as well and if any decrease, WACC will follow suit.
Market risk premium is used in the calculation of the cost of equity when using CAPM in the following manner:
= Risk free rate + Beta * Market risk premium
As can be inferred from the above formula, if the market risk premium declines, a lower cost of equity will result which would then reduce the WACC as well.