Select the correct passive form of the given sentence.

His father had already bought the train tickets.

The train tickets had already been bought by his father.

The correct passive form of the given sentence is The train tickets had already been bought by his father.

The active voice of the sentence is His father had already bought the train tickets. This sentence is in the past perfect tense, which is used to describe an action that happened before another action 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 "his father" and the main verb is "buy." The past participle of "buy" is "bought." So, the passive form of the sentence is The train tickets had already been bought by his father.

The other options are incorrect because:

• The train tickets were already bought by his father. is also in the past perfect tense, but it does not use the past participle of the main verb.
• The train tickets are already being bought his father. is in the present progressive tense, while the original sentence is in the past perfect tense.
• The train tickets have already bought his father. is in the present perfect tense, while the original sentence is in the past perfect tense.
• The train tickets already bought his father. is grammatically incorrect. The subject of the sentence is "the train tickets," but the action of buying is being performed by his father.