Final answer:
To convert the passive sentence into active voice, the subject that performs the action should lead the sentence. The correct active voice transformation of the provided sentence is option 3, which states 'The teachers believe that the school must implement a dress code to control behavior.'
Step-by-step explanation:
To change a sentence from passive to active voice, the subject performing the action should be placed at the beginning of the sentence. In the passive voice, the action is performed on the subject, and the sentence may include a form of the verb 'to be' along with the past participle of the main verb and can include 'by [noun phrase]'. However, in the active voice, the subject of the sentence performs the action directly.
For example, the passive sentence 'The vase was broken' can be changed to active voice by identifying the doer and constructing the sentence: 'Someone broke the vase.'
Using this understanding, the sentence 'It is believed by the teachers that a dress code must be implemented by the school to control behavior' is in passive voice.
The option that correctly changes the sentence into active voice by making the teachers the subject doing the action is: 'The teachers believe that the school must implement a dress code to control behavior.' Therefore, the correct choice to transform the given sentence into active voice is option 3.