。
She answered fluently.
Literal
She [topic-は] smoothly answered.
すらすら is a 擬態語 capturing smooth, effortless, flowing motion — applied equally to speech, reading, writing, or skating. It signals the absence of stumbling. Closely related mimetics for fluency or flow include ペラペラ ('jabbering / fluent in a foreign language'), さらさら ('silky / flowing'), and つるつる ('slippery / smooth surface'). 答える ('to answer') normally takes に for the question or person being answered; with neither mentioned, the sentence isolates the manner of the response.