I also agree that the answer is B: whom
in this sentence you are using the  preposition + whom grammar structure. Whom is considered an object pronoun and is used when asking who will receive the action, which in this case, I believe is, baking those cookies. Hopefully  this makes sense and good luck.  
 
        
                    
             
        
        
        
C and D aren’t the answer,which leaves A and B. I’m guessing the answer is B.