I'm pretty sure that they obtain shares in the company.
A share of a company is basically part ownership in the company. The more you own, the more you financially have put into the company by purchasing them.
A contract from the company wouldn't give ownership in the company.
A certificate of debt is another way of saying a bond, which isn't right. A bond is, "If you give me $100 today, I'll give you $105 in five years." It's basically borrowing money with an IOU.
The shares are the only answer that actually gives any ownership.
