CSP is used for making client-side callouts (this case)
RSS or Named Credentials are used for server-side callouts (using an Apex controller, for example)

B. The endpoint URL has been added to Remote Site Settings.

In Salesforce, when making an outbound call from a Lightning Web Component to an external endpoint, it is necessary to add the endpoint URL to the organization's Remote Site Settings. This allows Salesforce to make the callout to the external endpoint. If the callout is failing, the first thing an Integration Architect should validate is that the endpoint URL has been correctly added to the Remote Site Settings.

Great resource for this question:
https://roycon.com/how-to-make-rest-api-callouts-from-lightning-web-components/

The correct answer is A.

First, we need to add our Endpoint URL to CSP Trusted Site definition. This is a major difference between callout from Apex vs Callout from the LWC controller, As in Apex, we need to add RemoteSiteSetting. While in LWC JS Controller CSP trusted site do the work. If you just added it wait a few minutes before making callouts as it takes few minutes to reflect.