A sentence has been given in Active/Passive voice. Out of the four alternatives suggested below, select the one which best expresses the same sentence in Passive/Active voice. 
 

She could have done that work.

That work could have been done by her. 

That work could have been done by her.

Explanation:

• That work could had been done by her.: This construction is grammatically incorrect ("could had" should be "could have").
• That work could have been done.: This construction is correct but leaves out the agent (her) who could have done the work.
• That work could have been done by her.: This construction correctly maintains the original meaning and reverses the subject and object while including the appropriate agent (her).
• That work could be done by her.: This active construction changes the tense from past ("could have done") to present ("could be done"), which may alter the original meaning.