Explanation
Mailable Database usage is the number of prospects in an account that are opted in and can receive emails. It does not include prospects that are opted out, in the recycle bin, or have a hard bounce or do not email status.
Therefore, the Mailable Database usage for the marketer with 4,000 total prospects, 3,000 of which are opted in, 500 of which are opted out, and 500 of which are in the recycle bin, is 3,000 (A). The other options (B, C, D) are incorrect because they either include prospects that are not mailable or exclude prospects that are mailable. References: Marketing Cloud Account Engagement - Salesforce.com