Answer:
The comma is used in this sentence due to a participial phrase.
Explanation:
In the sentence "Completely exhausted by the hike, the climbers rested at the foot of the peak," "completely exhausted by the hike" is a participial phrase.
A participial phrase is formed by a participle (present or past) followed by pronouns, nouns, or phrases that complement what was said by the participle. That sort of phrase functions as an adjective, modifying a noun. In the case we are studying here, the participle is "exhausted" and it is modifying the noun "climbers", that is, it is attributing a characteristic/state to it.
Extra explanation: an adverb clause would function as an adverb, modifying an adjective, a verb, or another adjective. They can indicate manner, place, time, cause etc., and they must present a subordinating conjunction. For example: Because she gave me money, I went to the shopping mall.
An interjection has the purpose of conveying an emotion in an exclamatory way - "Wow!", "Ouch!" etc.
