If a partner withdraws a fixed amount at the end of each quarter, the average period for which interest is charged is calculated as: \[ \text{Average Period} = \frac{\text{Time from First Withdrawal + Time from Last Withdrawal}}{2}. \] Withdrawals occur at the end of: 1st quarter: Remaining time is 9 months. - 2nd quarter: Remaining time is 6 months. - 3rd quarter: Remaining time is 3 months. - 4th quarter: Remaining time is 0 months. For average period: \[ \text{Average Period} = \frac{9 + 0}{2} = 4.5 \text{ months}. \] Hence, the correct answer is (D).