The correct passive form of the given sentence is:

☀ The robbers escaped from the factory, but they were seen by a man.

Let's break down the options:

1. The robbers escaped from the factory, but they were seen by a man. (Correct) This sentence correctly uses the passive voice and conveys the same meaning as the original sentence. It uses the correct form "they were seen" to indicate that the robbers were observed by a man.

2. The robbers escaped from the factory, but they are seen by a man. (Incorrect) This option uses the present tense "are seen," which suggests an ongoing or habitual action. The original sentence is in the past tense, indicating a one-time event. The correct form should use the past tense "were seen."

3. The robbers escaped from the factory, but a man had seen them. (Incorrect) This option uses the past perfect tense "had seen," which suggests that the man saw the robbers before the robbers escaped. The original sentence does not indicate any temporal relationship between the robbers escaping and the man seeing them. The correct form should use the simple past tense "were seen."

4. The robbers escaped from the factory, but they are seeing by a man. (Incorrect) This option uses the present tense "are seeing," which is not appropriate for the context of the sentence. The passive voice should use the past tense "were seen" to match the active sentence. Additionally, "seeing" suggests that the robbers are actively observing something, which does not make sense in this context. The correct form should use "were seen" to indicate that the robbers were observed by a man.