Final answer:
The sentence uses a metaphor by presenting 'a trunk' as a symbol for a large amount of information, metaphorically stored in someone's office or knowledge base.
Step-by-step explanation:
The sentence "his office must contain a trunk of useful information" employs figurative language, specifically a metaphor. In this context, the word 'trunk' metaphorically represents a large storage area for information, suggesting that the person's office, or perhaps their knowledge, possesses a vast amount of information that can be useful. Instead of meaning a literal wooden chest, 'trunk' is used to conceptually illustrate the idea of a repository of knowledge.