The correct answer is C. Air travel is much more expensive than bus travel.
Step-by-step explanation:
In language, to contrast two or more elements, ideas, etc. means to explain the differences between them. This can be achieved by pointing out the features that make the two elements different, using contrasting transitions such as on the other hand, on the opposite, etc., or by using comparative structure to explain how an element is better, bigger, more complex, etc than the other. According to this, the sentence best contrasts two things is "Air travel is much more expensive than bus travel" because in this case, the author points out the difference between air travel and bus travel by comparing both in terms of price and stating one is much more expensive than the other.