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

☀ A nap was being taken by Suman.

So, the correct passive form is: "A nap was being taken by Suman."

The other options and their explanations:

☀ A nap has been taken by Suman.

• This option uses the present perfect tense ("has been taken"), which indicates an action that started in the past and is still relevant in the present. While grammatically correct, it does not match the past tense of the original sentence.

☀ A nap is being taken by Suman.

• This option is in the present continuous tense ("is being taken"), which does not match the past tense used in the original sentence.

☀ A nap was taken by Suman.

• This option uses the simple past tense ("was taken"), which is grammatically correct in the passive voice 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.