Re: The newly elected baseball commissioner has asked that a

The newly elected baseball commissioner has asked that a federal arbitrator would mediate negotiations between representatives of the umpire's union,which has threatened to go on strike,and the lawyers representing major league franchise owners

A. that a federal arbitrator would mediate negotiations between representatives of the umpire's union,which has threatened

B. that a federal arbitrator mediate negotiations between representatives of the umpire's union,which have threatened

C. of a federal arbitrator that he mediate negotiations between representatives of the umpire's union,which have threatened

D. a federal arbitrator that he mediate negotiations between representatives of the umpire's union,which has threatened

E. a federal arbitrator to mediate negotiations between representatives of the umpire's union,which has threatened

Can someone please explain me in this sentence which is modifying union or representatives, and how to identify it?