Also, even if there were a plug, I would count that as an item, and as it was not in the list, you are not allowed to chose it.

Right answer: Drink the tea, only a crazy pony would waste good tea.

“I would pull the plug.”  
“There is no plug. Obviously you’re crazy.”
 
Poor girl. It was a trap with no right answer, so the doctor could get ponies to experiment on. In a few weeks he’ll have her believing she’s a dog.

what plug? that tub doesn’t have a drain.

That “nurse” in the last panel…

I would’ve failed…

Corrections:
 
Panel #2: “You’re in charge of this place for crazy ponies. How do you check that somepony should be here?”  
Panel #4: “First, we fill this bathtub with herbal tea. Then we order our … let’s say … possible future patients to empty it.”  
Panel #5: “They can choose one of these items, which can help in their task. They can pick a spoon, a cup or a bucket.”  
Panel #6: “Right. A normal pony would use a bucket!”  
Panel #7: “No. A normal pony would pull the plug.”  
Panel #8: “Which room would you like? With or without a view?”
 
Also, woot, lateral thinking!

There’s no drain on your tub. Therefore, you are the crazy pony, doctor.

Who’s Normal Pony?  
I don’t see an a.