Which of the Following Conditions,if True,would Eliminate the Free Rider/collective

Which of the following conditions,if true,would eliminate the free rider/collective action problem?

A) if participation in an interest group were mandatory
B) if members were allowed to vote on matters of concern to the interest group
C) if participation in an interest group were voluntary
D) if the leaders of an interest group were able to set the group's priorities
E) if an interest group focused on providing benefits to society as a whole