Final answer:
Stress weakens the immune system, making individuals more susceptible to infections.
Step-by-step explanation:
The finding from the study where volunteers were given nasal drops containing the cold virus (Cohen et al., 1998) can explain why people seem to become sick during stressful times in their lives. The answer is a. Stress weakens the immune system, making individuals more susceptible to infections. When individuals experience chronic stress, especially related to work or relationships, their immune system is weakened, making them more likely to get sick when exposed to the cold virus.