Query Logs and evaluation of search engines
I would have selected option C.

I don't think option A is correct.  We probably can learn common typos, but that doesn't affect the time to process a search query, and it's not clear how you'd use that to speed up search queries.  (I'm assuming that "time to process a search query" is what is meant by "retrieval efficiency".)

Similarly, I don't think option B is correct.  We probably can learn the connection between different pages that a user clicked, but that doesn't appear to affect the time to process a search query, as far as I can tell.