Marcia is 80 years old and is learning how to speak french and play the guitar for the first time. this new skill development is a good example of

This new skill development is a good example of "plasticity."

Plasticity refers to the capacity of the brain to change and adjust to new data or information.  

Brain plasticity is the capacity of the mind to adjust its associations or re-wire itself. Without this capacity, any mind, not simply the human cerebrum, would be not able create from outset through to adulthood or recoup from mind damage.

answer; john b watson,(observable behavior);