Dr. Rojo conducted a longitudinal study on infant temperament, testing the same group of children at ages 4, 9, 14, 24, and 48 months. She found that exuberant infants did not change much, reacting the same way and having similar brain-wave patterns when confronted with frightening experiences every time they were tested. Inhibited, fearful infants, however, altered their responses as they grew older. This probably speaks to the influence of:

Child-rearing methods

Explanation:

 The child-rearing method is one of the type of parenting in which the it helps in developing the various types of emotional and also the children intellectual development.

The child rearing method is basically refers to the parenting technique that helps in determining the different types of relationship the parents have with their children.

According to the given question, Dr Rojo  is basically conducting the various types of longitudinal study that helps in influence the child rearing method based on the given situation.      

  Therefore, Child-rearing methods is the correct answer.

the density would be a smaller number.