We traveled . . . . . . . . the 6.45 train, which arrive at 8.30. A. by B. on C. in D. by

The correct answer is on.

We traveled on the 6.45 train, which arrived at 8.30.

The preposition on is used to indicate a means of transportation. In this case, the train is the means of transportation.

The other options are incorrect because they do not make sense in the context of the sentence.

• By is used to indicate a mode of transportation. In this case, the train is not a mode of transportation, it is a means of transportation.
• In is used to indicate a location. In this case, the train is not a location, it is a means of transportation.
• At is used to indicate a time. In this case, the time is already specified in the sentence.