The narrator stops suddenly when she comes outside and
sees the hurdles.

Is this referring to setting character or plot?

It refers to the setting

The setting would be the introduction of the narrator and or other characters and when and where the story takes place.

Explanation:

Things taking place leading to the plot of the story would include (when she comes outside and sees the hurdles.)

the answer is a because the astrophel does more and sidney goes to the astrophel more than stella.