I'm having trouble understanding this answer in a study plan test.  Could you please explain.

Ella estaba lavándose el pelo cuando él llegó.
She was washing her hair when he arrived.

When it is combined with another action that interrupts the ongoing action at that time. The interrupting action is generally in the simple past (see the last example where the ongoing action "She was washing her hair" was interrupted by another sudden action "he arrived”.

 Te ________ hasta que me aburrí y me fui.  
I was waiting for you until I got bored and left.

era esperando
estaba esperando-----my answer
estuve esperando------correct Kwiziq answer
fui esperando

 To me, it seems that the waiting was interrupted by "got bored and left."