The app builder should add the field to the custom report type field layout to troubleshoot this issue. A custom report type is a type of report that defines which objects and fields are available for reporting. The app builder can customize which fields are included in each custom report type by editing its field layout. If a new field is added to an object that is part of an unmanaged package, it will not be automatically added to the custom report type field layout. The app builder needs to manually add it to make it available as an option to add as a column in reports based on that custom report type. Option A, B, and D are not steps that can troubleshoot this issue.