An increase in a firm's tax rate will if the firm has debt capital in its capital structure: a. increase both the cost of preferred stock and debt.
b. decrease the cost of preferred stock.
c. increase the firm's WACC.
d. decrease the firm's WACC.

d. decrease the firm's WACC.

Explanation:

As per WACC formula

WACC = ( Weight of Common Equity x Cost of Common Equity ) + ( Weight of Common Debt x Cost of Common Debt x ( 1 - Tax rate ) ) + ( Weight of Preferred Equity x Cost of Preferred Equity )

By assuming the values to prove the answer

Weights

Common equity = 55%

Preferred Equity = 15%

Debt = 30%

Costs

Common equity = 15%

Preferred Equity = 8%

Debt = 12%

Tax rate is 15%

Placing values in the formula

WACC = ( 55% x 15% ) + ( 30% x 12% x ( 1 - 15% ) ) + ( 15% x 8% )

WACC = 8.25% + 3.06% + 1.2% = 12.51%

Keeping others values constant, Now increase the Tax rate to 25% and placing vlaues in the formula

WACC = ( 55% x 15% ) + ( 30% x 12% x ( 1 - 25% ) ) + ( 15% x 8% )

WACC = 8.25% + 2.7 + 1.2% = 12.15%

Hence the WACC is decreased from 12.51% to 12.15% when the tax rate is increased from 15% to 25% keeping other values constant.

a is the correct answer, tell me if im right