Final answer:
In 'The Scarlet Ibis,' the use of a first-person narrative, mature language, and reflective commentary indicates it is being told by the brother as an adult through the use of flashbacks. The correct option is A.
Step-by-step explanation:
The reader can tell that "The Scarlet Ibis" is being told when the brother is an adult mainly through the use of flashbacks (A). The narrative is conveyed from a first-person point of view, with the adult brother recounting the events from his past.
The mature tone of the language and perspective allows the reader to infer that the narrator is reflecting on past events, rather than experiencing them as they happen. Additionally, the story is written with hindsight and reflective commentary, which is characteristic of an adult looking back on their life. The correct option is A.