Final answer:
The phrase in question is a metaphor, as it directly compares Simon's anger to the behavior of a hornet without using 'like' or 'as'.
Step-by-step explanation:
A metaphor is a figure of speech that is used to make a comparison, but in a way different from a simile. Instead, it makes the description of an object look as if it was literally true. In other words, it can be said that a metaphor is an implied comparison.
The phrase 'Simon was mad as a hornet after discovering his bike had been stolen' uses a figure of speech that compares Simon's anger to that of a hornet, known for its aggressive behavior when provoked. This comparison is made without using the word 'like' or 'as'; therefore, it is not a simile, which would use such words.
Instead, this phrase is a metaphor, which is a figure of speech that directly refers to one thing by mentioning another for rhetorical effect, often providing a vivid comparison that allows us to understand the intensity of Simon's anger. The correct answer is (C) metaphor.