Change the following from active to passive:

The panel had been reviewing sensitive proposals when the alarm sounded.

The correct option is Sensitive proposals had been being reviewed by the panel when the alarm sounded.

To convert the given active sentence into the passive voice, follow these grammatical rules:

1. Identify the main clause and its structure:
Active sentence: "The panel had been reviewing sensitive proposals when the alarm sounded."
Subject: The panel
Verb Tense: Past Perfect Continuous (had been reviewing)
Object: sensitive proposals
Time Clause: when the alarm sounded (this dependent clause remains unchanged in structure).

2. Rules for converting Past Perfect Continuous tense into Passive Voice:
Active Form: Subject + had + been + V-ing + Object
Passive Form: Object + had + been + being + Past Participle (V3) + by + Subject

3. Apply the transformation:
- Move the object to the subject position: Sensitive proposals
- Apply the passive auxiliary verbs for Past Perfect Continuous: had been being
- Convert the main verb 'reviewing' to its past participle form: reviewed
- Add the agent preposition phrase: by the panel
- Retain the background time clause: when the alarm sounded

Combining these elements gives the complete passive sentence:
"Sensitive proposals had been being reviewed by the panel when the alarm sounded."