Lifespan developmentalists or developmental scientists are:

The options were a) mainly researchers. b) researchers and practitioners from a variety of disciplines. c) mainly psychologists. d) mainly practitioners. The answer is b) researchers and practitioners from a variety of disciplines. Lifespan developmentalists are the people who study the development in long term or we can say that in a lifespan. They examine different aspects like growth, change, behaviour development etc so as these all are from different disciplines it requires the researchers to be practitioners from a variety of disciplines.

The answer is: researchers and practitioners from a variety of disciplines.

developmental scientists include both physiological and psychological factors when analyzing the development in both humans and Animals.

Because of this, developmental science cover a huge variety of disciplines that contribute their unique perspectives to the analyzation