Hello there!
In the following sentece: "Matt excitedly jumped into his Dad's new pickup truck, while his mother was chary about getting in," we have two parts
1. Matt excitedly jumped into his Dad's new pickup truck
2. his mother was chary about getting in (the car)
And those two proposition are linked by a conjunction: while.
While can be used to make a contrast; it means, two things that are not similar .
This is not a relation cause and effect because there is not any result for the action of one of them; and, as they are not explaning anything that happend, it is no an explanation. Actually, there is a description, not an explanation.
Therefore, for all the reasons previously exposed, correct answer is B. Antonym.
Hope this helps.