The given sentence is in active voice: "The secretary has planted a lot of saplings to save the environment."

To convert it into passive voice, we need to make the object ("a lot of saplings") the subject of the new sentence. The verb needs to be changed to a passive form using a form of "to be" + past participle. The original subject ("The secretary") becomes the agent of the action, introduced by "by".

Let's analyze the options:

• Option 1: "A lot of saplings were planted by the secretary to save the environment." - This uses the simple past passive, which doesn't reflect the perfect aspect of the original sentence.
• Option 2: "A lot of saplings are planted by the secretary to save the environment." - This uses the simple present passive, incorrect for the original tense.
• Option 3: "A lot of saplings have been planted by the secretary to save the environment." - This correctly uses the present perfect passive, matching the tense and aspect of the original active voice sentence. The "have been planted" construction accurately reflects the completed action with ongoing relevance.
• Option 4: "A lot of saplings are being planted by the secretary to save the environment." - This uses the present continuous passive, indicating an ongoing action, which doesn't match the original sentence's meaning.

Therefore, only option 3 accurately reflects the original meaning in the passive voice.