The word that best completes the sentence is defiant.
The sentence would read: "Annika tried to get her little brother to bed, but he was defiant."
In this context, "defiant" is an adjective describing the brother's behavior as resistant or showing a refusal to obey.
Here’s how the other options fit into parts of speech and why they wouldn't work in this sentence:
- defiance (noun) - refers to the act of resisting.
- defy (verb) - means to openly resist.
- defiantly (adverb) - describes the manner of acting defiant.
Since the sentence requires an adjective to describe "her little brother," "defiant" is the correct choice.