According to Kurt Lewin, the changes which take place within a group are…

Kurt Lewin, regarded as the founder of the group dynamics movement in social psychology, viewed a group as a "dynamic whole" in which a change affecting any one member or subpart alters the state of the group overall. This continuous internal change and movement within a group is what is meant by its dynamism, the central idea behind Lewin's concept of group dynamics.

The question asks for Lewin's term for "the changes which take place within a group." Among the given options, only dynamism directly denotes internal change and movement. Reciprocity describes a mutual exchange relationship between members, not internal change; creativity describes a trait or ability to generate ideas, not change; and cohesiveness describes the degree of unity binding members together - the group's stability - not the change occurring within it.

This matches the standard definition used in psychology: group dynamics is described as the processes, forces, and changes operating within and between groups. The defining idea is change itself, which is exactly what dynamism denotes, confirming it as the keyed answer.