"Who" is the correct choice, but it is not possessive - that would be "whose".
Your suggestion of using "to who" is faulty grammar - this is never appropriate. In contrast, "to whoever" has limited correct usage, when "whoever" is not the direct object but the subject of a clause that is, in turn, the direct object of "to".
Also, why did you suggest replacing "whom" with "to who"? Putting the infinitive (or even the preposition) "to" anywhere near that sentence makes no sense.
The "sentence" has a weird grammar that can best be indicated as "World Cup [is] who?" Technically, the verb should not be omitted from a sentence, but this is spoken English.
An alternate way to look at the grammar is by removing "World Cup" and saying simply, "Who [is it]?" This grammar would be just as bad (two subjects for one verb, no conjunctions), but in a different way.
So, while your BS meter caught me red handed, you'd best hold off on becoming an English teacher for the nonce. :-P |