Final answer:
Option a. www.example.com is an example of a URL, which is the address of a webpage, while a DOI is a permanent identifier for digital documents.
Step-by-step explanation:
An example of a URL (Uniform Resource Locator) from the provided options is a. www.example.com. A URL is the address of a specific webpage or file on the Internet. Each URL is unique to the webpage it points to, allowing users to navigate directly to that page. In contrast, an example of a DOI (Digital Object Identifier) is not given in the options. Yet, it is important to know that a DOI is a permanent identifier used to reliably locate digital documents even if their URLs change, which is particularly common in academic and research settings for journal articles.