A.     who, whom

In formal English we use preposition + whom:

With whom did you go?     

To whom were you speaking?

But in ordinary English we usually move the preposition to the end of the sentence. The whom then normally changes to who:

Who did you go with?      

Who were you speaking to?

B.     which, what

In formal English we use preposition + which/what:

To which address did you send it?

On what do you base your theory?

In informal English we move the preposition to the end of the sentence:

Which address did you send it to?

What do you base your theory on?