Indeed, this is kind of like the Sesame Street game of "Which thing doesn't belong?" And you're right to say it's the swimming pool that doesn't belong, because the swimming pool is the only thing on this slide that's actually very dangerous.
De hecho, es como el juego de Plaza Sésamo de "¿Qué cosa no pertenece?" Y es correcto que respondan que la piscina no pertenece, porque la piscina es lo único de la imagen que es realmente muy peligroso.