Hi Neott

Thanks for clarifying above, now it is more clear

really appreciate

Thanks

  D is correct due to the unique feature of joint tenancy: Right of Survivorship.

If a joint tenant dies, the remaining joint tenants receives the deceased person's ownership.  Once a joint tenancy is created, a joint tenant cannot change this in his will.  However, while that joint tenant is still alive, he can transfer/sell his interest without the consent of other joint tenants.  But upon transfer, he severs the right of survivorship as well. 

Therefore, in this case,  Carla sells her interest to Diane while she's alive.  Diane becomes a 1/3 tenant in common, she will not have right of survivorship.  Right of survivorship remains with Astrid and Ben.

Next, Ben dies with a will, trying to pass his interest to his son, Evan.  But no way, that's not allowed by joint tenancy.  Ben should have done that when he was alive.  As a result, lucky Astrid got 2/3 interest thanks to right of survivorship.  
        

thanks for the great explaination