If a website’s search engine saturation with respect to a particular search engine is 20%, what does it mean?

a. 20% of the webpages of the website have been indexed by the search engine
b. Only 20% of the pages of the website will be indexed by the search engine
c. 20% of the websites pages will never be indexed
d. The website ranks in the first 20% of all websites indexed by the search engine for its most important             search terms