B. Protocol identifier

Explanation:
A Uniform Resource Locator (URL) contains several components, and the protocol identifier (such as “http” or “https”) is a mandatory part of the URL. It defines the method used to access the resource over the internet. Without it, the web browser wouldn’t know how to retrieve the requested resource. Other components like the file name or port number are optional in most cases. Therefore, the correct answer is B. Protocol identifier.