Field Theory is the contribution of:

Concept — A school of psychology is defined by the unit it treats as basic when explaining behaviour. Gestalt psychology takes the organised whole as that unit: a psychological event is determined by the structure of the total situation in which it occurs, not by the sum of its isolated parts. Field theory is the direct extension of that principle from perception to behaviour — behaviour is treated as a function of the person together with the whole psychological environment acting on that person at that moment, written B = f(P, E).

Application — Field theory was developed by Kurt Lewin, who worked in the Berlin Gestalt tradition alongside Wertheimer, Kohler and Koffka. He carried the Gestalt idea of the organised whole from perception into motivation and social behaviour, naming the total set of coexisting facts that determine a person's behaviour at a given moment the life space, or psychological field. Because both the founding principle and its author belong to Gestalt psychology, Field Theory is credited to the Gestaltists.

Cross-check — Among the four schools offered, only one takes the whole organised situation, rather than an element, a stimulus-response bond or an unconscious drive, as its unit of explanation, and the field in field theory is exactly that whole. Field Theory is therefore the contribution of the Gestaltists.