The best word to complete the sentence is defiant.
Using it in context: "Annika tried to get her little brother to bed, but he was defiant."
In this case, "defiant" is an adjective that describes her brother's attitude (i.e., resisting authority or control).
The other options do not fit grammatically:
- "defy" is a verb.
- "defiance" is a noun.
- "defiantly" is an adverb and would need to modify a verb or adjective.
So the complete sentence is: "Annika tried to get her little brother to bed, but he was defiant."