What's the reading of 空く?

すく or あく? What's the difference?

Answer

Both.

Many words in Japanese have multiple readings, sometimes with different meaning or nuance. In this case, the meanings are rather different, but you have to infer the reading from context. すく means "to be empty" and あく with that kanji most often means "to be available" in terms of time or space.

お腹が空{す}いている。

"I'm hungry." (literally: "My stomach is empty.")

[何時]{いつ} 空{あ}いていますか。

"What time are you available?"

However: the あく reading can also mean "to be empty". The nuance isn't always clear to me as a non-native speaker, but 空{す}く often implies "not crowded."