Answer:
A new world record was set by Michael Stone.
Explanation:
In the passive voice, the object of the sentence (in this case, "a new world record") becomes the subject. The subject of the sentence in the active voice (in this case "Michael Stone") moves to the end of the sentence in the passive voice, preceded by the word "by". The verb form changes to the past participle ("set"). Thus, "Michael Stone set a new world record" turns into "A new world record was set by Michael Stone" in the passive voice.