Opitamal capital strucutre problem. I am having the hardest time understanding optimal capital structure. A great explanation would help tremedously when doing this problem. Thank you in advance.

Understanding the optimal capital structure Review this situation: Transworld Consortium Corp. is trying to identify its optimal capital structure. Transworld Consortium Corp. has gathered the following financial information to help with the analysis WACC 9.71% 9.55% 10.02% 10.78% 11.45% Debt Ratio Equity Ratio rd 30% 40% 50% 60% 70% 70% 60% 50% 40% 30% 6.02% 6.75% 7.15% 7.55% 8.24% 9.40% 9.750% 10.60% 11.30% 12.80% Which capital structure shown in the preceding table is Transworld Consortium Corp.'s optimal capital structure? Debt ratio = 70%; equity ratio = 30% Debt ratio = 30%; equity ratio-70% Debt ratio 50%; equity ratio 50% Debt ratio 40%; equity ratio 60% Debt ratio 60%; equity ratio 40%