Final answer:
The sentence uses a simile.
Step-by-step explanation:
The sentence "If we wake him too early, our dad is a grumpy bear." is an example of a simile. A simile is a figure of speech that compares two unlike things using "like" or "as". In this case, our dad being compared to a grumpy bear using "is" indicates that it is a simile.
Learn more about Understanding similes and metaphors