Answer: the "sky"
Explanation: abstract nouns are the ones that aren't concrete or show a concept, an idea, something intangible. Being like this, you cannot experience this "thing" using your five senses, and maybe that's where the concept gets twisted. You can "see" the sky, right? But what you see exaclty are the elements that compose the sky, not the sky itself, because "sky" is an idea. What you look at exactly are the clouds and the sun, that are made of real matter. That's why "sky" is the abstract noun you are looking for in the sentence.
Ps: an easier way to think, if you are still wondering why is "sky" the abstract noun, is that when you look at it and you can't see the sun, clouds or the moon, for example, you wouldn't recognize it as the "sky" we know. In this way, once again we can see that sky is the idea behind a group of concrete things gathered together.