No two learners are alike in height, strength, sensory sharpness, reasoning power, interest, attitude or temperament. Psychology groups all such unlikeness between one person and another, whether bodily or mental in origin, under one technical heading, and the question is which heading that is.

The variation among individuals on measurable bodily and psychological traits is the subject matter of a well established branch of the discipline. Skinner described it as the variation or deviation among individuals in regard to a single characteristic or a number of characteristics, and its study underlies ability grouping, remedial teaching and the whole idea of teaching according to the learner.

The other choices name different concepts. The organised whole of a person's traits is a description of one individual, not of the unlikeness between several. Fitting oneself to the demands of surroundings is a process, not a variation. A bare word for unlikeness is too general and is not the technical term used in psychology.

Personal differences option is correct.