Final answer:
The past tense 'transgressed' correctly completes the sentence, as it fits the context that indicates the actions occurred in the past.
Step-by-step explanation:
The correct form of the word to complete the sentence 'He often ____ the school code of conduct and was frequently in detention.' is transgressed. The sentence is in the past tense, as indicated by the context 'was frequently in detention.' Therefore, the past tense of the verb is required.
When conjugating the verb 'transgress,' which means to violate a rule or law, we need to match the tense with the rest of the sentence. To convey that the action happened regularly in the past, we use the simple past tense 'transgressed,' which implies this was habitual behavior.