What is the reason for the tension that exists between Willy and his son Biff?

I can take b; it's a very interesting question, by the way.  Nothing in the play tells us directly why Willy thinks that Biff will be a huge success.  But indirectly, we know that Willy believes firmly in the American dream--and part of that dream is that children will be more successful their parents.  Willy's brother Ben, for instance, fulfilled this aspect of the dream, and Willy thinks that if he raises his sons right that they will become as successful as Ben.  Willy, however, focuses mainly on Biff, not Happy.  Biff is the one with the charisma, the high school football hero, the one that the girls liked. He was, unlike Bernard, "well liked," at least that is what both Willy and Biff believed.  So much of Willy's dream is tied to Biff.  If Biff succeeds, and success is measured in terms of money, then Willy can feel validated.  He can rest knowing that he has provided well for his sons, that he has raised them well and that even if he is a failure as a salesman and as a husband, he can be successful as a father.