Final answer:
The correct passive form of 'The workers were digging a tunnel' is 'A tunnel was being dug by the workers,' which is option A.
Step-by-step explanation:
The sentence "The workers were digging a tunnel" can be changed to passive voice as follows: "A tunnel was being dug by the workers." This version of the sentence is passive because the subject (a tunnel) is not performing the action but is receiving it. The suitable answer from the provided options is A) A tunnel was being dug by the workers. In the passive voice, the object of the active sentence becomes the subject of the passive sentence, and the subject of the active sentence is optionally indicated by the preposition 'by' followed by the noun phrase (the workers in this example).