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Correct answer: B. Memory, which picks up useful context from conversations without being told to
Explanation
The principle — ChatGPT has two personalisation features and they differ by who supplies the information.
Why the key is correct — OpenAI draws the line explicitly: for explicit information or instructions, you add it to your custom instructions, and for information shared via conversations, ChatGPT can remember relevant details for you. A marathon mentioned in passing is information shared via conversation, so memory is what carried it into the new chat.
Why the others are wrong — the custom instructions field contains only what you typed there; it does not absorb things you say mid-chat. History is a list of conversations you can reopen rather than context supplied automatically to a new one. And a project's knowledge is scoped to that project, so a chat started outside it sees nothing.
Remember this — you write custom instructions, ChatGPT writes memory. If you are surprised that it knows something, memory is the first place to look, and the memory summary in settings is where you can read, correct or remove it.
Sources — OpenAI's article on memory versus custom instructions, and the Memory FAQ.
Sources
“For explicit information or instructions, you can add it to your Custom Instructions. For information shared via conversations, ChatGPT can remember relevant details for you.”
“You can see the information that ChatGPT remembers in the memory summary.”
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