The correct answer is: C. 6 months.
If the withdrawals are made by the partners evenly in the beginning of each month, interest is usually calculated for the whole of the amount (annual) for 6 months. This is because the interest is calculated on the principal amount, which is the amount of money that is borrowed. When the partners withdraw money from the account, the principal amount decreases. This means that the interest that is calculated is based on a smaller amount of money, so the interest is also smaller.
Option A is incorrect because the interest is calculated for the whole of the amount (annual) for 6 months, not 5 months.
Option B is incorrect because the interest is calculated for the whole of the amount (annual) for 6 months, not 5 and half months.
Option D is incorrect because the interest is calculated for the whole of the amount (annual) for 6 months, not 6 and half months.
Option E is incorrect because the interest is calculated for the whole of the amount (annual) for 6 months, not an average of 12 months.