As a customer's account receivable balance is specifically written-off due to its being uncollectible, what is the effect on the company's total assets amount?

A decrease or reduction

Explanation:

The account receivable balance represents the amount yet to be collected from a customer after revenue has been earned. It is an asset, a current asset in the balance sheet.

As such, when an amount becomes uncollectible, it is written off to the p/l as an expense resulting in a reduction in the accounts receivables and thus a reduction in the company's total asset amount.