Among the options provided, d) nontaxable entity is not a characteristic of a corporation.
To elaborate, a corporation is recognized as a separate legal entity, which means it is distinct from its owners. This allows for a) separate legal entity status, providing limited liability for its owners, where their personal assets are protected from the debts and obligations of the corporation.
Moreover, while ownership in a corporation can be flexible, especially in terms of transferring shares, this is not what makes a corporation unique as a business structure. Therefore, c) flexible ownership can be seen as a characteristic of corporations, albeit not a defining one.
However, corporations are not nontaxable entities. In fact, they are subject to corporate taxes on their profits, making d) nontaxable entity the correct answer as it does not apply to corporations.