Today, most psychologists think that a. late adulthood begins before the age of 50 years old. b. cognitive functions start to decrease during early adulthood. c. people continue to develop psychologically during adulthood. d. social and emotional health normally decline during adulthood.

C.  People continue to develop psychologically during early adulthood.

Explanation:

This is correct on gradpoint.  And..

late adulthood is defined as 60+

Although some people can experience loss of cognitive function in early adulthood it is not common and usually part of a disease process.

Social and emotional health could decline during adulthood but again not commonly therefore not the best (truest) statement.

this is true my friend.