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Correct answer: C. Custom instructions cover this, and they work on every plan and device
Explanation
The principle — the most useful personalisation feature in ChatGPT costs nothing, which makes it the first thing a new user should set.
Why the key is correct — OpenAI states that custom instructions are available on all plans on Web, Desktop, iOS and Android. On the web you find them under Settings and then Personalization; on the phone apps, under Settings and then Customize ChatGPT.
Why the others are wrong — it is not a paid upgrade, since it is available on every plan. It is not web-only, because the same field exists in the mobile apps. And it is a personal setting rather than something an administrator applies on your behalf.
Remember this — free accounts get this too, so set it on day one.
Sources — OpenAI's ChatGPT Custom Instructions article.
Sources
“Custom instructions are available on all plans on Web, Desktop, iOS, and Android.”
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