Select the option that expresses the given sentence in passive voice.

Someone stole his traveller's cheques when he was travelling in Europe.

His traveller's cheques were stolen when he was travelling in Europe.

The correct passive form of the given sentence is His traveller's cheques were stolen when he was travelling in Europe.

The active voice of the sentence is Someone stole his traveller's cheques when he was travelling in Europe. This sentence is in the past simple tense, which is used to describe an action that happened in the past. The passive voice of the sentence is formed by putting the subject of the active sentence after the verb "to be" and the past participle of the main verb. In this case, the subject of the active sentence is "traveller's cheques" and the main verb is "steal." The past participle of "steal" is "stolen." So, the passive form of the sentence is His traveller's cheques were stolen when he was travelling in Europe.

The other options are incorrect because:

• His traveller's cheques are stole when he was travelling in Europe. has a typo. The past participle of "steal" is "stolen," not "stole."
• Someone had been stole his traveller's cheques when he was travelling in Europe. is in the past perfect tense, which is used to describe an action that happened in the past before another action happened. There is no other action mentioned in the sentence, so the past perfect tense is not appropriate.
• His traveller's cheques was stole when he was travelling in Europe. is in the singular form, but the subject of the sentence is plural.