Which of the following pairs is an example of an antonym relationship?

The pair "Happy : Sad" exemplifies an antonym relationship because these two words express opposing emotional states. "Happy" denotes a feeling of joy and positivity, whereas "Sad" represents a feeling of sorrow and negativity. The fundamental contrast in their meanings clearly establishes them as antonyms.

In contrast, "Tree : Leaf" describes a part-to-whole relationship, where a leaf is a component of a tree, rather than opposites. "Light : Dark," while closely related, forms a different type of antonym pair, as it also represents opposing concepts of illumination and absence of light, but is not the chosen correct answer. Lastly, "Car : Vehicle" represents a category relationship, with a car being a specific type of vehicle, demonstrating a familial connection rather than opposition in meaning.