Unfortunately, the correct answer is "whoever", which doesn't appear in your list!
Where the relative pronoun "who(m)ever" acts as both the object in one clause, and the subject in another, you use "whoever"; where it acts as the object in both clauses, you can use "whomever", e.g.
1. I'll vote for whoever is willing to offer a substantial bribe.
— here, you need a subject for "is willing"; thus "whoever".
2. I'll vote for whomever you prefer.
— here, the relative pronoun is the object of "for" and the object of "prefer"; thus "whomever".
However, "whomever" is extremely rare in current English; the tendency is to use "whoever" even in sentences of the #2 kind.