Which of the following best describes a web crawler?

A web crawler, often referred to as a spider or bot, is primarily designed to browse the internet in an automated fashion to gather information about websites. This process involves scanning and indexing web pages, allowing search engines to understand and categorize the content they contain. By doing so, web crawlers ensure that search engines like Google can provide relevant search results to users based on the indexed data.

The function of a web crawler is distinct and crucial to the overall process of search engine operation, as it enables the indexing of vast amounts of content available on the internet. This, in turn, improves the accuracy and speed of search results when users enter queries.

While the other options relate to different functions or tools in the realm of web technology, such as website performance measurement, design, and optimization, they do not pertain to the specific functionality of a web crawler. A web crawler's main job is focused solely on content indexing, which sets it apart from these other functionalities.