Answer: Down
Step-by-step explanation:
In the given sentence, the word "down" is the preposition that fills in the blank.
A preposition is a word that shows the relationship between a noun or pronoun and other words in a sentence. In this case, "down" is used to indicate the direction or movement of the ball. It shows that the ball is moving from a higher position to a lower position along the stairs.
Here, "down" is an appropriate preposition because it accurately describes the action of the ball rolling in a specific direction. Other prepositions that could have been used in this context include "along" or "towards," but "down" is the most suitable choice based on the given sentence.
So, the correct answer is: The ball rolls down the stairs.