Answer:
The sentence that best conclude this paragraph is
D) Taking a little time to plan can result in happier travelers.
Step-by-step explanation:
Because a good conclusion sentence is the one that retakes the topic sentence (A multi-day road trip can be a pleasant experience with a little preparation.) in order to reafirm the central idea of the paragraph. In this case preparation and planning are related actions which appears at the beginning and in the end.