The given passage is a narration of events that have already taken place, as suggested by the past tense verbs 'could', 'knew', 'hadn't eaten', etc.
Hence, the correct verb form will be in the past tense.
Option A: 'Exhausted' is a past participle (verb form in the past) that describes the person, Rusty, as exhausted. It is the most suitable option here, hence A is correct.
A participle is a verb form that functions as an adjective (describing a noun), rather than a verb (describing an action).
Option B: 'Exhausting' is a present participle that refers to the current time. It isn't the appropriate option here as the event is in the past.
Option C: 'Exhaust' is a noun, hence it cannot describe another noun. Nouns do not modify other nouns.
Option D: 'Exhausts' is a verb in the simple present tense, whereas the passage indicates the past.
Hence options B, C and D are incorrect.