Definitions of learning differ in the word each writer stresses. Some stress modification through experience, some stress reorganisation of thought, and one well known definition stresses that the adaptation of behaviour keeps moving forward step by step, which is exactly what the quoted sentence says.

That progressive wording belongs to the American behaviourist who built the theory of operant conditioning. Working with reinforcement schedules, he saw behaviour as being shaped in small successive approximations, so for him learning was a process of progressive behaviour adaptation. His emphasis on shaping is the reason the word progressive appears in the statement.

The other names are linked with different formulations. One educational psychologist described learning as modification of behaviour through experience and training. Another explained growth of knowledge through assimilation, accommodation and stages of thought rather than through shaping. The last name is not associated with any standard definition of the process.

Skinner option is correct.