Marie visits her neighbor's house along with her 7-month-old son, Noah. When she places Noah on the floor of her neighbor's house, he crawls away from her and starts exploring nearby objects. However, he occasionally glances at Marie to ensure that she is close by. In the context of socioemotional development, which of the following concepts is exemplified by Noah's behavior?
a) secure attachment
b) object permanence
c) conservation
d) assimilation