Cost of Capital - Cost of Debt

Cost of debt is one of the main inputs when calculating the weighted average cost of capital. Which of the following is the correct cost of debt in the weighted average cost of capital calculation?

()The yield to maturity on a company's debt.

()The yield to maturity on a country's debt.

()The after-tax yield to maturity on a company's debt.

()The coupon rate on a company's debt.