The correct passive form of the given sentence is:

☀ By whom was it bought?

Let's break down the options:

1. By whom was it bought? (Correct) This sentence correctly uses the passive voice and conveys the same meaning as the original sentence. It uses the correct form "was it bought" and includes "by whom" to indicate the agent in the passive construction.

2. It was bought by who? (Incorrect) This option uses "who" instead of "whom." In the passive construction, "whom" is used as the object of the preposition "by," not "who."

3. Who was it bought by? (Incorrect) This option changes the word order and uses "who" instead of "whom." The correct form should use "whom" as the object of the preposition "by," as shown in option 1.

4. It was taken by whom? (Incorrect) This option changes the verb to "taken," which is not the correct form of the verb "buy" used in the original sentence. The original sentence is about "buying" something, not "taking" something. The correct passive form should use "bought," as shown in option 1.