Right Answer is:
The given sentence is of past perfect tense and it is in active form. The structures for active/passive voices are:
Active : Subject + had + verb (IIIrd form) + object…
Passive: Object + had + been + verb (IIIrd form) + by + subject…
So, the passive voice of the given sentence would be:
The train had been boarded by them after booking the tickets.
Hence, option C is the correct answer.