Explanation:

- The sociologist who identified four major types of co-operation—spontaneous, traditional, directed, and contractual—is Robert Nisbet.

- Lewis Coser is known for his work on the functions of social conflict, not specifically on types of co-operation.

- George Simmel contributed theories about social forms and conflict, but not this classification.

- George C. Homans focused on social behavior and exchange theory, not categorizing cooperation as such.

- Robert Nisbet outlined the four types of co-operation:

- *Spontaneous*: Natural and unplanned co-operation.

- *Traditional*: Based on customs and traditions.

- *Directed*: Organized by an authority.

- *Contractual*: Based on formal agreements.

Option:3, Robert Nisbet is the correct answer.