The best word to complete the sentence is "defiant."
The completed sentence would read: "Annika tried to get her little brother to bed, but he was defiant."
"Defiant" is an adjective that describes her brother's behavior. The other options are not suitable in this context:
- "Defiance" is a noun.
- "Defiantly" is an adverb.
- "Defy" is a verb.
Thus, "defiant" is the correct choice.