It is imperative that ________ anonymity of the whistle-blower ________ maintained, as even ________ slightest leak could compromise the entire federal investigation.

Concept (Mandative Subjunctive with Articles) After 'It is imperative that' the Mandative Subjunctive is used, which takes the base form of the verb, so 'anonymity ... be maintained' is correct. In 'The anonymity of the whistle-blower' the specific reference needs the, and in 'even the slightest leak' the superlative 'slightest' takes the.

✓ Correct sentence It is imperative that the anonymity of the whistle-blower be maintained, as even the slightest leak could compromise the entire federal investigation.

Examiner's Trap 'Is maintained' seems normal, but after imperative the that-clause takes the base form; placing a before 'slightest' is another trap.