Final answer:
The default sharing setting for the Campaign Member object to allow visibility based on user access to associated campaigns is 'Controlled by Parent'. It ensures users only see campaign members for campaigns they have permission to view.
Step-by-step explanation:
The organization-wide default sharing setting that can be used for the Campaign Member object to allow all users to see only the campaign members associated with the campaigns they have access to is Controlled by Parent. This setting means that the user's access to a campaign member is determined by their access to the parent campaign. If a user has permission to view a particular campaign, they will be able to view the members of that campaign. On the other hand, if they do not have access to a campaign, they will not see its associated members. This sharing setting helps maintain data security and integrity by ensuring that users only see the data relevant to campaigns they are authorized to access.