The correct answer is Meg was found to be the better behaved of the four.
Explanation:
The passive voice is used to emphasize the action. That is to say, it does not matter so much <em>WHO </em>performed the act, but <em>WHAT </em>act was performed.
To form the passive voice it is necessary to use the subject + auxiliary verb (to be) + past participle.
In this case:
Meg <em>was </em><u><em> </em></u><u>found</u> to be the better behaved of the four.