A developer receives a 401 Unathorized HTTP status message from a REST API request. What is the reason for this error?

A 401 Unauthorized error indicates either the absence of an access token in the request or that the provided token is invalid or has expired, necessitating a re-authentication.

A. Lack of privileges usually results in a 403 Forbidden error, not a 401.

C. OAuth is supported for API resources; if not, it would be a different HTTP error.

D. Too many requests typically result in a 429 Too Many Requests error, not a 401.

- Salesforce Marketing Cloud API Error Codes Documentation, pp. 5-6.

- OAuth 2.0 RFC 6749, Section 5.2, pp. 28-29.

- REST API Error Handling Guidelines, pp. 2-3.

- Stanford University Course on Web Security, "API Authentication and Error Codes," pp. 10-11.