D. To encourage party members to vote with party leaders.
Explanation:
The task of a party whip is to make that the members of the party get behind the same proposal and they all vote the same. They must assure discipline within the party in the legislature. They must convince some of them to adopt the party´s ideology and attend the voting sessions.
They are both right because Barack Obama was the one who elected the justices into the Supreme Court but they are of course in no way connected to Barack Obama except for the fact that he was the one who elected them into the court.
