Answer:
A noun that works as a subject.
Explanation:
Actions need someone or something that performs them, that is to say, a noun, a doer. Without a noun that performs the main action of "ran", this sentence leaves room for doubt and confusion. So, in order to avoid ambiguity, this sentence needs a noun that works as a subject that performs the main action. In this case, to fill the blank with the pronoun "They" or any other noun in third-person plural would be right.
