The correct option that expresses the given sentence in passive voice is:

☀ Banners were being prepared by them with different designs for the sports meet.

So, the correct passive form is: "Banners were being prepared by them with different designs for the sports meet."

The other options and their explanations:

☀ Banners have prepared by them with different designs for the sports meet.

• This option is in the present perfect tense ("have prepared"), which does not match the past tense used in the original sentence.

☀ Banners had been prepared by them with different designs for the sports meet.

• This option is in the past perfect tense ("had been prepared"), which indicates an action that was completed before another past action. It is grammatically correct, but the original sentence is in the simple past tense.

☀ Banners were prepared by them with different designs for the sports meet.

• This option is in the simple past tense ("were prepared"), which is grammatically correct and conveys the same meaning as the original sentence. However, it does not include the continuous aspect of the action as expressed in the original sentence.