The app builder should add the fields to the page layout on the object to ensure users can access the new object and fields. The page layout determines which fields are visible and editable on the record detail page. Option A, B, and C are not necessary for this purpose.

To import opportunities with the proper country currency, the app builder needs to include the currency ISO code in all currency fields in the import file, and use Data Loader to import the records. Data Loader supports importing multiple currencies, while Import Wizard does not. The currency ISO code column in the import file specifies the currency for each record

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.

A text-only Visualforce page can be used to show a text notification on a case record page based on a condition. The Visualforce component can be dragged into the Case Lightning page and its visibility can be set to show when the account support level is platinum. This is the simplest and most flexible way to meet the requirement

A report type that is in a status of In Development is not available for users to select when creating a new report. The app builder needs to change the status to Deployed to make it visible to users