Final answer:
Extensible cheek pouches are a characteristic found specifically in some species of monkeys. These adaptations are used for storing food and are not found in birds, rodents, or reptiles.
Step-by-step explanation:
Among the groups mentioned, monkeys are known to have developed extensible cheek pouches. This feature is particularly found in some species of the Cercopithecoidea superfamily, which includes the Old World monkeys like the baboons and macaques. These cheek pouches are used to store food while foraging and are a useful adaptation in environments where competition for food is high. While birds, rodents, and reptiles have unique adaptations specific to their evolutionary paths, extensible cheek pouches are not among them. For instance, reptiles, such as crocodiles and lizards, display adaptations suited for their ecological niches, like scaly skin to prevent water loss and specialized breathing mechanisms, but not extensible cheek pouches. Therefore, the correct answer to the question 'Extensible cheek pouches are adaptations found in?' is 1) Monkeys.