What is the primary cause of the error message "Access to fetch at from origin has been blocked by CORS: No 'Access-Control-Allow-Origin' header is present on the requested resource"?
A) The requested resource does not exist on the server.
B) The server does not include the 'Access-Control-Allow-Origin' header in its response.
C) The client-side application has a syntax error in its code.
D) The server is down and not responding to requests.