Change (Active Voice to Passive Voice) Barnaby had not forgotten the sugar-coated explosives.

Answer: If you need it in Passive Voice, it will be:

The sugar-coated explosives had not been forgotten by Barnaby.

Explanation: Passive Voice always needs the verb to be plus Past Participle of the verb. This sentence in the Past Perfect Tense Active Voice, so to change it to Past Perfect Passive Voice, we need to switch subjects, ans=d then add the verb to be in Past Perfect (it is "Had been"), and then add the actual verb (forget) in Past Participle (forgotten).