C. Separation anxiety disorder
Separation anxiety is a disorder of childhood that is most often seen in children 7-8 years old. The disorder manifests itself as an overwhelming fear of loss (usually of a parent) and can lead to physical complaints to avoid going to school or other activities. Separation anxiety is a normal part of development that occurs between 8-18 months; however, in a child >5 years old, separation anxiety that affects normal activity is described as a disorder.
Childhood schizophrenia (option A) develops before puberty and presents as withdrawal, failure to develop a separate identity from the mother, and may include autistic-like behaviors.
Conduct disorder (option D) is a persistent behavior that manifests as a violation of the rights of others (stealing, fighting, malicious acts). In those older than 18 years, this disorder is classified as antisocial personality disorder (option B).