Explanation
Named Credentials and Protected Custom Settings are two persistence mechanisms that can be used to ensure that secrets are protected from deliberate or inadvertent exposure. Named Credentials allow you to specify the URL of a callout endpoint and its required authentication parameters in one definition. Salesforce manages all the authentication for Apex callouts that specify a named credential as the callout endpoint, and you don't have to add more authentication logic in your Apex code. Named Credentials can be defined to provide a secure and convenient way of setting up authenticated callouts, and they can also be used in Lightning components, Visualforce pages, and flows1.
Protected Custom Settings are a type of custom settings that store application-specific data that is hidden from subscribers. They are only accessible by the managed package that created them, and they can be used to store secrets such as encryption keys, passwords, or tokens. Protected Custom Settings can be accessed by Apex code, formulas, or validation rules within the same namespace as the settings2.
Encrypted Custom Fields are not a suitable mechanism for storing secrets, because they are not designed to prevent unauthorized access to sensitive data. Encrypted Custom Fields allow you to encrypt text fields using a standard encryption scheme. The encrypted data is masked in reports, list views, and search results, but it can still be viewed by users who have the "View Encrypted Data" permission. Encrypted Custom Fields are intended to protect data from unauthorized access by users within your organization, not from external threats or malicious code3.
Protected Custom Metadata Types are another type of custom metadata types that store application-specific data that is hidden from subscribers. They are similar to Protected Custom Settings, but they have some advantages such as being deployable using change sets or Metadata API, being accessible by SOQL queries, and being able to reference other metadata types or settings. However, Protected Custom Metadata Types cannot be used to store secrets, because they do not support encryption or masking of sensitive data. Protected Custom Metadata Types are intended to store configuration data that is specific to your managed package, not secrets that need to be secured.
Therefore, the correct answer is B and D, because Named Credentials and Protected Custom Settings are the only persistence mechanisms that can be used to securely store secrets in Salesforce.
References: 1: Named Credentials | Apex Developer Guide | Salesforce Developers 2: Custom Settings | Apex Developer Guide | Salesforce Developers 3: Encrypted Fields | Salesforce Help : [Protected Custom Metadata Types | ISVforce Guide | Salesforce Developers]