To troubleshoot the issue of a contact being unavailable for other users to see, an app builder should check the following things:
Confirm whether Default Organization-Wide Sharing Settings provide access to the Account. This setting determines the baseline level of access that users have to each other's records.
Review the Contact record and ensure it is linked to an Account. Contacts that are not linked to an account are private and only visible to the owner and administrators.
Verify the users with the issue have access to the Contact object. Users need to have at least read permission on the Contact object to view contact records. Option A and E are not relevant for troubleshooting this issue.