The loss was already recognized when the allowance was estimated. Writing off identifies which account it belonged to.
That is the direct write-off method, which United States GAAP does not permit for material amounts because it recognizes the loss in the wrong period.
Net receivables do not move, because the allowance falls by the same amount as the gross balance.
A recovery reinstates the receivable and then records the collection. Nothing about it is revenue; the sale was recognized long before.
Answer: a
Choice a. Expense moves when the allowance is estimated, not when a specific account is written off.