You want to ask about a personal medical worry, and you would rather the conversation left no trace in your history and shaped nothing later. What do you use?

The principle — for one sensitive conversation, choose the mode before you type rather than tidying up afterwards.

Why the key is correct — OpenAI states that temporary chats do not show in history, do not use or create memories, and are not used to train models, and that they are deleted from OpenAI's systems after 30 days. That covers every part of what you asked for, and it covers it from the first message.

Why the others are wrong — deleting an ordinary chat afterwards leaves a window in which it sat in your history and could inform memory. A project is an organising tool with memory of its own, not a privacy boundary. And the training toggle is the closest wrong answer: it stops conversations being used to improve the models, but OpenAI states plainly that your conversations will still appear in your chat history.

Remember this — the training setting is about OpenAI's models; temporary chat is about your account. When the requirement is that nothing is kept and nothing is learned, that is temporary chat.

Sources — OpenAI's What is ChatGPT FAQ and Data Controls FAQ.