When using Data Import Wizard, which of the following data types is NOT directly supported?

Correct answer: Attachments

Option D is correct because the Salesforce Data Import Wizard does not support Attachments as an importable object type; attachments and files must be loaded through the Data Loader or other API-based tools. Option A is wrong because Leads are one of the core standard objects explicitly supported by the Data Import Wizard. Option B is wrong because Accounts and Contacts are also directly supported and can even be imported together with relationship matching in a single wizard session. Option C is wrong because Custom Objects are supported by the Data Import Wizard, provided they have been created in the org, making them available in the object selection list.