The correct answer is Radha is greeted cheerfully by them every morning.

The original sentence is in active voice, with the subject "They" and the verb "greet". To change it to passive voice, we make the object of the active sentence, "Radha", the subject of the passive sentence. The verb "greet" becomes "is greeted", and we add the preposition "by" to show who is doing the action. This gives us the passive voice sentence "Radha is greeted cheerfully by them every morning".

The other options are incorrect.

• Every morning Radha was greeted cheerfully is in the past tense passive voice. This sentence is not appropriate for the original sentence, which describes an action that happens every morning.
• Radha is being greeted cheerfully by them every morning is in the progressive passive voice. This sentence is used to describe an action that is happening in the present. The original sentence does not describe an action that is happening in the present.
• Cheerful greeting is done by them every morning to Radha is not in passive voice. The subject of the sentence is "Cheerful greeting", which is not a person. This sentence is not grammatically correct.