Final answer:
Section 166 allows William to claim a bad debt deduction for an account receivable if it is partially or completely uncollectible, given he has included it in income under the accrual method and determines it is uncollectible.
Step-by-step explanation:
Under §166 of the Internal Revenue Code (IRC), related to the accrual method of accounting, a taxpayer like William can claim a bad debt deduction for an account receivable when it becomes either partially or wholly uncollectible. If William has included the amount in his income previously and has made a reasonable determination that the debt is uncollectible, he is allowed to write it off and claim a deduction. He must have a substantiated and reasonable belief that some or all of the receivables won't be collected.
Therefore, out of the options provided, 1) and 2) are correct as William can claim a bad debt deduction for the account receivable if it is either partially uncollectible or completely uncollectible.