The correct answer is: The plants have been watered by the gardener.
This is because the sentence uses the past participle “watered” as the main verb. The past participle is used to form the passive voice, which means that the focus of the sentence is on the action, not on the person or thing doing the action. In this case, the focus is on the fact that the plants have been watered, not on the gardener who watered them.
The other options are incorrect because they use the present tense or the past tense, which would focus on the gardener rather than the action. Option A, “The gardener is watering the plants,” would mean that the gardener is currently watering the plants. Option B, “The gardener has been watering the plants,” would mean that the gardener has been watering the plants for some time. Option C, “The gardener has watered the plants,” would mean that the gardener watered the plants at some point in the past.
In conclusion, the correct answer is: The plants have been watered by the gardener. This is because the sentence uses the past participle “watered” as the main verb, which is used to form the passive voice. The passive voice focuses on the action, not on the person or thing doing the action.