Option C is the option that makes the most sense.
The phrase: "spring out of her fingertips" most likely means that the music was played so well and in an effortless manner.
Let's substitute each option with the phrase from the sentence.
Option A:
"And what do you know—Chopin's waltzes and her fingers look like they're dancing and she's the most surprised thing in the world."
The option/phrase doesn't quite match with the sentence.
Option B:
And what do you know—Chopin's waltzes are like she created the music on her own and she's the most surprised thing in the world.
This also doesn't math the context of the sentence, so it doesn't make any sense.
Option C.
And what do you know—Chopin's waltzes are played effortlessly, and she's the most surprised thing in the world.
Option C matches the best with the sentence, because the music seemed like it burst out of her fingers because she played it well/with no effort.
Option D:
And what do you know—Chopin's waltzes and her fingers move up and down quickly, and she's the most surprised thing in the world.
This option also doesn't match with the context. The author could've used other types of figurative language to describe options A,B and D.
Option C is the correct answer.
I hope this helps! o(〃＾▽＾〃)o