Answer:
You're right, it's pronoun.
Explanation:
Stamina is not a "direct object," or "adjective" nor is it a "subject."
Definition of pronoun says "a word that can function by itself as a noun phrase and that refers either to the participants in the discourse (e.g., I, you ) or to someone or something mentioned elsewhere in the discourse (e.g., she, it, this )."
