Explanation:

- Original Statement: Every passing vehicle was being thoroughly checked by the guards.

- Passive voice structure indicating an ongoing action in the past.

- Option 1: The guards were thoroughly checking every passing vehicle.

- Correct. This is the active voice mirroring the original meaning.

- Option 2: The guards have thoroughly checked every passing vehicle.

- Incorrect. This implies the action was completed, which does not match the ongoing nature.

- Option 3: Every passing vehicle were thoroughly checking the guards.

- Incorrect. Subject-verb agreement error, and meaning is reversed.

- Option 4: The guards have been thoroughly checking every passing vehicle.

- Incorrect. This suggests an action starting in the past and continuing to present, but isn’t parallel to "was being."

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