Ellie the dog yelps in fear whenever she hears thunder. the fact that ellie also yelps when she hears fireworks is best explained by however, the fact that she does not yelp when she hears a loud truck is best explained by

The correct answers are Stimulus generalization and Stimulus Discrimination.

Stimulus Generalization refers to the possibility for the conditioned stimulus to respond to generalized stimulus, in the given situation, it is the thunder. While Stimulus Discrimination  refers to the ability ti differentiate between a conditioned stimulus and another stimuli, which has no pairs, which is why in the situation, Ellie the dog does not yelp on loud trucks.