The correct Active/Passive form of the given sentence is:

☀ Where was the red bag found?

Explanation for other options:

☀ Where were the red bag founded? This option is not grammatically correct. The correct form of the verb in the passive voice should be "was found," not "were founded."

☀ Where have been the red bag founded? This option is not grammatically correct. The correct form of the verb in the passive voice should be "was found," not "have been founded."

☀ Where had been the red bag founded? This option is not grammatically correct. The correct form of the verb in the passive voice should be "was found," not "had been founded."