Public corporation is the innovation of the 20th century. Its basic feature is:

Public corporations are a significant innovation of the 20th century, representing a unique organizational form distinct from traditional government departments and private companies. Their basic feature stems from the government's expanding role in economic and social sectors.

• The core reason for the emergence of public corporations is It is the result of government’s entry into business (C). As governments began to provide essential services like utilities, transportation, or industrial production, they needed entities that could operate with commercial efficiency and autonomy, yet remain accountable to the public. Public corporations were created by statute to fulfill these specific commercial or public service functions.
• A corporation by courtesy (A) is not a recognized feature; it's too vague.
• While they are indeed Created for particular purpose (B), this is a characteristic shared by many organizations, not the fundamental feature explaining their origin in the context of government involvement in business.
• It has virtue of business management (D) describes an *objective* or *advantage* of public corporations—to operate efficiently—rather than their fundamental defining feature or the reason for their creation.

Therefore, their existence fundamentally signifies the government's direct engagement in business-like activities.