Explanation: \(\text{Hints: } f_1\lambda_1 = f_2\lambda_2; \, f_2 - f_1 = 200 \\ \text{Solve: এখানে, সুপরিকল্পিত দুটি একক মাধ্যমের সূত্রানুযায়ী, } v_1 = v_2 \\ \implies f_1\lambda_1 = f_2\lambda_2 \implies f_1\lambda_1 = f_1 \cdot 3\lambda_2 = f_2\lambda_2 \\ \implies \lambda_2 = 3\lambda_1 \, \text{বা, } f_2 - f_1 = 3f_1 - f_1 = 200 \, \text{Hz} \\ \therefore f_1 = 100 \, \text{Hz, } f_2 = 300 \, \text{Hz} \\ \text{Ans. (C)}\)