The Tamil sentence means that the gardener would have poured water on the plants before the sun sets. Its verb ‘ஊற்றியிருப்பார்’ (ஊற்றி + இருப்பார்) is a perfect form with a future/presumptive sense: ‘would have poured’ or ‘will have poured’.

All four options are in the passive voice with ‘the plants’ as subject. The passive of ‘would have watered’ is ‘would have been watered’, which keeps both the completed action and the presumptive sense of the Tamil verb.

‘Will be watered’ is simple future and loses the completed-action meaning; ‘are being watered’ is present continuous; ‘had been watered’ is past perfect, describing an action completed in the past rather than one presumed to be completed.

The plants would have been watered by the gardener before sunset option is correct.