The correct option is 404. This status code indicates that the server could not find the requested resource. It is a standard response for situations where the server cannot locate the requested URL on the server.

Negative testing involves sending invalid types of data to check how well the API handles unexpected inputs. Valid data is used in positive testing, not negative testing.

The access token in OAuth is primarily meant to identify the user who authorized the application, allowing the application to access resources on behalf of that user. It does not necessarily identify the client or resource server.