What might the moose population look like on Island Royale 10 years from now?

The answer to this question would be: b. constricting the blood vessels

Vasopressin can constrict the arteriole and reduce urine production by increasing the absorption in the renal tubule. When arteriole blood vessel constricts, the diameter of the vessel will be reduced. A smaller diameter will result in an increased peripheral resistance and cause the blood pressure to be increased. 
Vasopressin doesn't affect the blood natrium level.