When a partner makes a fixed withdrawal at the close of each quarter, the average interest-charging period is determined by the formula: \[ \text{Average Period} = \frac{\text{Time from First Withdrawal + Time from Last Withdrawal}}{2}. \] The withdrawal timing and remaining periods are as follows: - End of 1st quarter: Remaining time is 9 months. - End of 2nd quarter: Remaining time is 6 months. - End of 3rd quarter: Remaining time is 3 months. - End of 4th quarter: Remaining time is 0 months. Applying this to calculate the average period: \[ \text{Average Period} = \frac{9 + 0}{2} = 4.5 \text{ months}. \] Therefore, the correct option is (D).