A developer is asked to find a way to store secret data with an ability...

When a requirement involves storing configuration or "secret" data that needs to vary based on the specific user or profile, Hierarchy Custom Settings (Option C) are the correct choice. Unlike Custom Metadata (Option B), which is available to the entire organization regardless of the user, Hierarchy Custom Settings use a built-in logic to provide values based on the "most specific" level defined.

The hierarchy follows a specific order: User > Profile > Organization. If a secret key is defined at the User level, that value is returned for that specific user. If not, the system looks for a value defined at the user's Profile level, and finally falls back to the Organization-wide default. This allows a developer to store sensitive keys or flags and restrict or vary them dynamically based on the current user's identity.

Custom Metadata is better suited for application-wide configurations that need to be deployable via packages, but it lacks the granular per-user/per-profile override capability inherent to Hierarchy Custom Settings. Static Resources (Option A) are public to anyone with access to the resource, and Cookies (Option D) are stored on the client side, making them insecure for "secret" data. Therefore, Custom Settings provide the best balance of security and hierarchical flexibility for this use case.

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