Answer:
Ahora.
Explanation:
The best time expression for this sentence is "ahora" because the sentence is in present progressive. So we need an expression of present time.
"I'm listening to music."
"Ahora estoy escuchando música."
Therefore, the sentence would sound odd with another time expression of past time:
*Last year I'm listening to music.
The sentence "estoy escuchando música" is an action performed at the moment.
