Final answer:
The condition characterized by excessive concern over having a serious illness despite medical reassurance is Hypochondriasis, also known as Illness Anxiety Disorder. Option a.
Step-by-step explanation:
The mental health condition that is characterized by excessive worry about having a serious illness despite medical reassurance, often accompanied by bodily symptoms, is A) Hypochondriasis. This condition is now more commonly referred to as Illness Anxiety Disorder in the DSM-5. It falls under the category of anxiety disorders, which are the most common mental disorders in the United States, affecting many adults annually. Unlike occasional anxiety which is a normal reaction to stress, anxiety disorders consist of persistent feelings of worry and fear that last for months, impacting one's quality of life and ability to perform daily activities.