Rehana's grand father is preoccupied with thoughts of developing a serious illness and constantly worries about this possibility. He is continually anxious about his health. He may be suffering from

Illness anxiety disorder

Illness anxiety disorder involves persistent preoccupation about developing a serious illness and constantly worrying about this possibility. This is accompanied by anxiety about one’s health. Individuals with illness anxiety disorder are overly concerned about undiagnosed disease, negative diagnostic results, do not respond to assurance by doctors, and are easily alarmed about illness such as on hearing about someone else's ill-health or some such news.

In general, both somatic symptom disorder and illness anxiety disorder are concerned with medical illnesses. But, the difference lies in the way this concern is expressed. In the case of somatic symptom disorder, this expression is in terms of physical complaints while in case of illness anxiety disorder, as the name suggests, it is the anxiety which is the main concern.

Therefore, the most appropriate answer is illness anxiety disorder, as Rehana's grandfather's excessive worry about developing a serious illness is a key symptom of this disorder.