Final answer:
According to the diathesis-stress model, with a high diathesis for a disorder but low stress levels, an individual will less likely develop a disorder.
Step-by-step explanation:
When considering what happens when diathesis is high but stress level is low, it's important to understand the diathesis-stress model. This model suggests that while a predisposition to a disorder (diathesis) may exist, the actual development of a disorder also depends on the presence of stress. With a high diathesis, the likelihood of developing a disorder may be increased but is not guaranteed without significant stress. Therefore, if stress levels are low, the individual will less likely develop a disorder, making option a. The individual will less likely develop a disorder the correct choice among the presented options.