The WACC is used in evaluating newly proposed capital budgeting projects. It should therefore be calculated using capital component costs and a capital structure:

a$.$

based on the capital on the firm's books because that's the capital it already has and will use to support new projects.

b$.$

based on conditions the firm will encounter when raising new capital in the next year because that's the capital it will use to support new projects.

c$.$

based on existing capital on the books because both old and new projects have to be supporteD$)$

d$.$

based on existing equity but new debt amounts and costs.