A trial balance exists to check that total debits equal total credits before the final financial statements are drawn up. That check is only meaningful once every transaction for the period has been recorded, which happens at the close of the accounting period, not on any fixed daily, monthly, or quarterly schedule. So the standard timing for preparing a trial balance is at the end of the accounting period.
We can also place the trial balance correctly by locating it within the accounting cycle's sequence of steps.
Placed in the cycle this way, the trial balance's position is fixed at the close of the period, not at any shorter interval.
Therefore, the correct answer is At the end of accounting period.