Answer:
The underlined pronoun in the sentence "The gorilla and the chimpanzee ran until one of them got tired." is referring to "one of" the gorilla and the chimpanzee.
(Choice A) "The animal" would not be a clear choice as it does not specify which of the two animals the pronoun is referring to.
(Choice B) "The gorilla" could be a possible choice if the sentence is modified to suggest that the gorilla was the one who got tired, but as written, the sentence does not specify which animal got tired.
(Choice C) "He" is not a clear choice as it does not specify which of the two animals the pronoun is referring to, and could also be interpreted as referring to either the gorilla or the chimpanzee.
In conclusion, the sentence as written is not clear, and additional information would be needed to determine the specific reference of the pronoun "one of them."