Fill in the gap with interjection sound: _________The Lion is sleeping now.

The Correct answer here is Hush.

Hush 
- It is an interjection.
- It is used as a command to be silent or quiet.

On the other hand, 
- Fie!  it shows disgust or disapproval. 
- Tut! it expressing disapproval, contempt, etc.
- Ahem! expressing gentle utterance used to call someone's attention.

Source: Cambridge Dictionary Grammar Preference.