Which of the following sentences is grammatically correct?

Correct option is D

The correct sentence is option (d) He went to college on a train.
Explanation: With public transport, we commonly use “on” to indicate the specific vehicle we are travelling in: on a bus/train/plane. We also use “by” to express the general means of travel without an article: by train/bus/air. Hence, “on a train” is correct here, while “by a train” is incorrect because “by” should not take an article in this usage.
Grammatical rule used:
· Prepositions with transport:
· by + mode (no article): by train, by bus, by air.
· on + the/a + vehicle: on the bus, on a train, on the plane.
· in + small/private vehicles: in a car/taxi/cab.
· Example: I go to work by train , but today I’m on a train heading to Delhi.
· Information booster / exceptions: We say “on foot”, not by foot; and with bicycles both on a bicycle and by bicycle are accepted depending on focus (specific vs. means).
· Why the other options are wrong:
· (a) in a train — unnatural with large public transport; we prefer on.
· (b) with a train — with denotes accompaniment, not means of transport.
· (c) by a train — article misuse; should be by train (no article).