Change the voice: He will have known it by this time.

He will have known it by this time- এই sentence টি Future Perfect Tense এ আছে। 
• Future Perfect Tense এর Active voice কে Passive voice এ রূপান্তর করার নিয়ম: 
- Object টি Subject হবে + shall/will + have been + Verb এর Past participle form বসবে + by + subject টি object হবে + phrase/clause বসবে। 
- তাই সঠিক উত্তরটি হবে: It will have been known by him by this time. 

Source: A Passage to the English Language by S M Zakir Hussain.