Approximately 40 percent of babies can be classified as , a type of temperament resulting in babies who are relatively happy, can be soothed when upset, and are quick to adjust to changes in the environment.

easy babies                                          

Explanation:

Easy baby: In psychology, the term "easy baby" has been described by many psychiatrists, for example, Chess and Thomas between 1987 and 1991, who believed that there are three different types of temperament associated with a child including easy, slow to warm up, and difficult.

The "easy baby or child" is generally considered as possessing positive mood, tends to establish regular routines quickly during the infancy period, and adjust to new experiences easily.

In the question above, the given statement signifies "easy babies".

answer/// playing;