This domain of development is socio-emotional. Around 14-18 months, toddlers experience seperation anxiety when they are away from their parents. This is when the toddler is in great distress when the parents is gone. As toddlers grow older, they are able to make more than one attachment besides their parents. Soon, the seperation anxiety subsides enough that the parent can leave and the toddler learns to cope until the parent is back.
Source of Image: