The word that best completes the sentence is "defiant."
So the complete sentence would read: "Annika tried to get her little brother to bed, but he was defiant."
"Defiant" is an adjective that describes someone who is resistant or disobedient, which fits the context of the sentence.
The other options do not correctly fit the grammatical structure of the sentence:
- "defiantly" is an adverb,
- "defy" is a verb, and
- "defiance" is a noun.
Therefore, the correct answer is defiant.