Answer: d. carpool instead of driving separately
Step-by-step explanation:
The best option is the one that leaves the least carbon footprint on the planet.
If they ride the bus instead of walking, the bus would burn more fuel (carbon) to carry them as opposed to if they simply walked.
If they drive separately instead of taking the train, the two cars would cause more pollution than if they took a train.
If they ride the train instead of their bikes, the train would burn more fuel or use more electricity to carry them as opposed to them simply using bicycles which do not cause pollution when used.
If they however carpool instead of driving separately, they would decrease their level of pollution because one car would cause less pollution than two.