The answer is: B) cost.
The tense of the clause is Present Perfect Simple, which is formed with the auxiliary verb has or have and the past participle. In this case, the verb cost is irregular - it does not take the past simple or past participle form with -ed. Actually, the past simple and the past participle forms of the verb cost do not change - the form remains the same: cost (infinitive)-cost (past simple)-cost (past participle).