The correct answer is option 4: "The department has been reorganised over the last year."

To understand why, let's analyze the transformation from active to passive voice. The original sentence, "We have reorganised the department over the last year," is in the active voice. The subject ("We") performs the action ("reorganised"). In passive voice, the object of the active sentence ("the department") becomes the subject of the passive sentence. The verb changes to a form of "to be" + past participle ("has been reorganised"). The original subject ("We") is often omitted in the passive voice, but could be added with a "by" phrase (although it is not needed here).

Let's examine why other options are incorrect:

• Option 1 ("Last year, the department was reorganised.") uses the simple past tense, which doesn't convey the ongoing nature of the action implicit in "over the last year".
• Option 2 ("The department is being reorganised over the last year.") uses the present continuous, indicating an action happening now, not an action completed over a period in the past.
• Option 3 ("The department is reorganised by me.") is grammatically correct passive voice but lacks the temporal element "over the last year". It also unnecessarily specifies the actor.

Option 4 accurately reflects the completed action ("reorganised") over a specified period ("over the last year") using the present perfect passive ("has been reorganised"), perfectly capturing the meaning and tense of the original sentence while shifting to passive voice.