Choice "D" is correct. The optimal capital structure is the mix of financing instruments that produces the lowest WACC.
Choice "a" is incorrect. The opposite is true. A low WACC would indicate to potential shareholders that the company is being managed to produce the highest stockholder value.
Choice "c" is incorrect. The opposite is true. The mixture of debt and equity securities that produce the lowest WACC maximizes the value of the company.
Choice "b" is incorrect. The company's borrowing rate is a component of the WACC. Be careful of always, all, never, etc., answer choices. These answers are usually incorrect answer choices.