Native British English speaker here, and I'm sure they are all wrong.
A.I bought the book for which you asked.
This is the most convincing, as it sounds like a very formal construction of "I bought the book you asked for". However, there are different meanings of the word "for", and the formal construction leans towards it meaning 'on behalf of'. In other words, it sounds like the person did some asking on behalf of the book, rather than actually requesting the book be bought. I can't imagine even the queen would say this.
B.Who that you had ever seen is stronger than him?
Just plain wrong.
C.The man and the horse which fell into the river were drowned.
Incorrect use of "which". It should be "the man and the horse that fell into the river were drowned". There is some ambiguity in this statement (were the man and horse together?) but ambiguity doesn't mean ungrammatical.
D.This is the place where we visited last year.
If you use 'visit' to mean you travelled to a single destination (not matter how large that destination is), you should say "this is the place that we visited last year. Chiefly in US English, 'visit' is sometimes used intransitively to mean an ongoing period of visiting different places, so it is possible this might be heard, but it definitely sounds wrong to a British English speaker.