The correct passive form of the given sentence is:

☀ The robbers were arrested by the cop after getting information.

 Let's break down the options:

1. The robbers was arrested by the cop after getting an information. (Incorrect) This option uses the incorrect verb form "was" with the plural subject "the robbers." The correct form should use "were" to agree with the plural subject.

2. The robbers arrested by the cop after getting an information. (Incorrect) This option is not in the correct passive voice construction. It lacks the appropriate form of the verb "to be" and the past participle "arrested."

3. The robbers were arrested by the cop after getting an information. (Correct) This sentence correctly uses the passive voice and conveys the same meaning as the original sentence. It uses the correct form "were arrested."

4. The robbers had arrested by the cop after getting an information. (Incorrect) This option uses the past perfect tense "had arrested," which is not appropriate for the context of the sentence. The past perfect tense suggests an action completed before another past event, but the original sentence is in the simple past tense, indicating a sequence of events without specifying any temporal relationships between them. The correct passive form should use the simple past tense "were arrested," as shown in option 3.