The correct passive form of the given sentence is:

☀ The previous project report has been asked by the students.

Let's break down the options:

1. The previous project report has been asked by the students. (Correct) This sentence correctly uses the passive voice and conveys the same meaning as the original sentence. It uses the correct form "has been asked."

2. The previous project report have been asked by the students. (Incorrect) This option uses the incorrect verb form "have been asked." The subject "The previous project report" is singular, so the correct form should be "has been asked," not "have been asked."

3. The previous project report were been asked by the students. (Incorrect) This option uses the incorrect verb form "were been asked." The correct form for the present perfect tense in the passive voice is "has been asked," not "were been asked."

4. The previous project report was been asked by the students. (Incorrect) This option uses the incorrect verb form "was been asked." The correct form for the present perfect tense in the passive voice is "has been asked," not "was been asked." Additionally, the subject "The previous project report" is singular, so the correct form should be "has been asked," not "was been asked."