Marginal cost of capital is basically addition to the cost due to the new project. WACC is for all projects running by company. If MCC and WACC both are given in the question, I would say choose MCC as MCC is more appropriate for choosing the new project, especially when “optimal” word is given in the question. WACC might not yield the correct decision as it could be either very low or very high. However, most of the time you undertake only those projects which are of same risk so WACC and MCC should ideally be same in general