This question tests your understanding of active and passive voice in English grammar. The first sentence, "Dust the furniture thoroughly," is in the active voice. The subject ("You," which is implied) is performing the action (dusting) on the object (the furniture).

The second sentence, "The furniture is _____ thoroughly," requires the passive voice. In passive voice, the object of the active sentence becomes the subject, and the focus shifts to the action being received rather than performed. The structure for the present continuous tense in passive voice is:

$\mathrm{am}/\mathrm{is}/\mathrm{are}+\mathrm{being}+\mathrm{past}\mathrm{participle}$

Let's analyze the options:

Step 1: Identify the tense and voice needed. The verb "is" indicates the present tense, and the blank requires a passive construction because the furniture is receiving the action.

Step 2: Evaluate the options:

• being dusted: This correctly follows the structure for present continuous passive voice ($\mathrm{is}+\mathrm{being}+\mathrm{dust}\left(\mathrm{past}\mathrm{participle}\right)$).
• dusted: This is the simple past tense or past participle alone, which does not fit with the helping verb "is" to form the required continuous passive.
• to be dusted: This is an infinitive passive form, which is not used after "is" in this context.
• be dusted: This is the base form and is grammatically incorrect after "is".

Final Answer: The most appropriate word is being dusted. The complete sentence is: "The furniture is being dusted thoroughly."

Related Topic: Active and Passive Voice

Active Voice means the subject of the sentence performs the action.
Example: The chef (subject) cooks (action) the food (object).

Passive Voice means the subject receives the action.
Example: The food (subject) is cooked (action) by the chef (agent).

The passive voice is formed using a form of the verb "to be" (am, is, are, was, were, been) + the past participle of the main verb.

Key Formulae (Structures)

Present Simple Passive: $\mathrm{am}/\mathrm{is}/\mathrm{are}+\mathrm{past}\mathrm{participle}$
Present Continuous Passive: $\mathrm{am}/\mathrm{is}/\mathrm{are}+\mathrm{being}+\mathrm{past}\mathrm{participle}$
Past Simple Passive: $\mathrm{was}/\mathrm{were}+\mathrm{past}\mathrm{participle}$