Final answer:
The statement that NGOs are usually understood to be part of the 'third sector' is not credited to a specific author from the options provided, yet the concept is well-established in civil society theory.
Step-by-step explanation:
The concept that NGOs (Non-Governmental Organizations) are usually understood to be part of the 'third sector' opposite to business and government operational orbits has been discussed by various authors, but the specific statement is not attributable to a single author based on the options given. However, the understanding of NGOs as a separate sector is commonly accepted in civil society theory. The term 'third sector' is used to describe the space occupied by non-profit organizations, charities, and NGOs, which are independent of both the government and the private sector.
It's significant to note that, while the NGO as a 'third sector' concept is widely recognized, the detailed history and analysis of the roles and operations of NGOs can be found in recent literature such as 'NGOs: A New History of Transnational Civil Society' by Thomas Davies, published by Oxford University Press in 2014.