Middle adulthood is
- marked by feelings of chaos and crisis.
- a contemporary phenomenon, basically nonexistent before the twentieth century.
- hard to define because few variations in attitudes and behaviors exist from early adulthood.
- more about aging mentally than physically.

Option C.

Explanation:

Middle adulthood (or midlife) refers to the period between the age of 45 to 65 years.

Middle adulthood is is a stage which is hard to define because there are few  variations in attitudes and behaviors exist from early adulthood.

There are several biological changes during this age which affects attitudes and behaviors of an individual. Physical changes involves aging which is inevitable; cognitive changes involves decline in intelligence and continued its descent; and great emotional upheaval, anxiety, and drastic changes in behavior with psychological changes.

Hence, the correct option is C.

where is the

answer; acquired mainly during early childhood;