Which of the following is considered a somatoform disorder?

A somatoform disorder is characterized by the presence of physical symptoms that cannot be fully explained by a medical condition, and are believed to be linked to psychological factors. Hypochondriacal disorder, now more commonly referred to as illness anxiety disorder, involves an excessive preoccupation with having a serious illness despite medical evaluations showing no significant health issues. This fits the definition of a somatoform disorder since the individual's psychological distress manifests in the form of physical concern without identifiable physiological causes.

In contrast, generalized anxiety disorder involves excessive worry about various aspects of life, but does not primarily manifest as unexplained physical symptoms. Major depressive disorder focuses on mood and emotional states, while oppositional defiant disorder is a behavioral condition that involves having an angry or irritable mood along with argumentative and defiant behavior. None of these disorders fit into the somatoform category since they do not primarily exhibit the psychological component that leads to physical complaints as seen in illness anxiety disorder.