Answer:
a. Index
Step-by-step explanation:
Web Indexing is the permanent exploration that Internet search engines perform. As the word itself indicates, search engines create their own index of all the content they are able to track. Each time a user performs a search in a search engine, this searches its index in order to deliver the result it considers best.
The reason why search engines create this index has to do with their ability to react. That is, having an index of their own allows them to respond quickly to the user.