Step 1: Understanding informal organization. 
  
An informal organization refers to the network of relationships that naturally forms among people within a formal structure, based on social and personal interactions. It is highly dependent on the willingness of employees to participate and work together informally. 
Step 2: Analyzing the options. 
  
(A) Formal organisation: This is incorrect. A formal organization relies on established rules and procedures, and is not dependent on the willingness of employees. 
  
(B) Functional organisation: This is incorrect. While a functional organization is structured by roles and departments, it doesn't depend on the willingness of employees. 
  
(C) Informal organisation: Correct. Informal organizations rely on the willingness of employees to cooperate and communicate beyond formal roles and structures. 
  
(D) Divisional organisation: This is incorrect. A divisional organization refers to a structure based on divisions, such as geography or product line, and is not dependent on the willingness of employees. 
Step 3: Conclusion. 
  
The correct answer is (C) Informal organisation, as it is driven by the willingness of employees to interact and work together informally.