The correct answer is b. debajo. Your sentence, "La alfombra está ____ de la mesa", says "The carpet is ____ the table." Adentro wouldn't make sense because that means "in", and "de la mesa" means "of", which also does not make sense. "debajo" means "under", which makes sense. "The carpet is under the table." OR "La alfombra está debajo de la mesa." Hope this helps! Have a great day! :)