Members called the majority whip and the minority whip assist the majority and minority party leaders in both houses. what is their role?

Members called the majority whip and the minority whip assist the majority and the minority party leaders in both housed to apply pressure and persuasion to keep party members in the chamber in line especially in creating support for their party's legislation.  
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A. to apply pressure and persuasion to keep party members in the chamber in line. 

Whips are a party's "enforcers", who typically offer inducements and threaten party members to ensure that they vote according to the official party policy

Democracy and darwinism would be my answer.