Final answer:
The financial product that makes Robyn a shareholder in a corporation is preferred stock, which represents ownership in the company and often entitles the holder to dividends.
Step-by-step explanation:
Robyn, being an investor, would become a shareholder in a corporation if she purchased preferred stock. Preferred stock is a type of equity that represents ownership in a corporation and often pays dividends. Unlike a corporate bond, Treasury bill, or Certificate of Deposit (CD), which are forms of debt and do not confer ownership, preferred stock provides Robyn with a claim on earnings and assets of the company, thus making her a shareholder.