When psychology first emerged as a science, its focus was on but its focus shifted between the 1920s-1960s when emphasis was placed on

When it first emerged, its focus was on mental processes but its focus shifted between the 1920s-1960s when emphasis was placed on observable behavior.

Explanation:

When Psychology first emerged as a science, its main focus was what we know as the mental processes; since scientists began to realize that there was more to the body than its physical form.

Such was the main study focus of Wilhelm Wundt or Sigmund Freud, with Psychoanalysis, for example.

As time went by, new studies and perspectives emerged, primarily with John Watson and Skinner, who introduced Behaviorism; and centered on the study of observable behaviors.

Nowadays even neuroscience has come into play.

i think it is the third one.