The correct answer is C. Two people had different predispositions for the disorder and had likely experienced different levels of stress during past encounters with similar stimuli.
Step-by-step explanation:
The diathesis-stress model proposes disorders related are the result of the interaction between stressful situations, and other factors in the individual such as genetics, psychological factors and previous experiences that make a subject more/less likely to develop a disorder if he or she goes through a stressful situation. Additionally, in this model, it is necessary the situation, and factors exceed the threshold of the individual, which leads to a disorder.
According to this, the fact only one of two subjects develops a disorder despite the two were exposed to the same stressful situation occurs because the two subjects had different predisposition in terms of genetics and experiences with stress, and it is likely the subject that developed the disorder was more predisposed or vulnerable to it (Option C).