The correct passive form of the given sentence is:

☀ The exercise has already been done by us.

Let's break down the options:

1. Already the exercise has been done by us. (Incorrect) This option changes the word order and places "already" at the beginning of the sentence, which is not the typical word order for passive constructions. The correct form should have "already" after "has been done."

2. The exercise has already been done by us. (Correct) This sentence correctly uses the passive voice and conveys the same meaning as the original sentence. It places "already" in the correct position after "has been done."

3. The exercise had been already done by us. (Incorrect) This option uses the past perfect tense "had been done," which implies that the action was completed before another past event. The original sentence is in the present perfect tense, not the past perfect tense, so option 2 is the correct choice.

4. The exercise is already done by us. (Incorrect) This option uses the simple present tense "is done," which changes the meaning of the original sentence. The present tense suggests a general truth or habit, whereas the original sentence talks about a specific past action that is already completed using the present perfect tense.