The best word set to complete the analogy is "caboose : train."
In the analogy, "engine" is a part of a "car," just as a "caboose" is a part of a "train." The relationship reflects a component to whole relationship, where the first term is a key element of the second term.
The other options do not share this kind of relationship in the same way:
- "song : singer" suggests a creator relationship, but not a part-whole relationship.
- "limb : tree" is a part-whole relationship, but it isn't as directly relevant to the context of vehicles.
- "sails : boat" could also be a part-whole relationship, but "caboose : train" is more analogous to "engine : car."
So the answer is caboose : train.