Final answer:
The factor called the c. moderating factor is significant in program design for health outcomes, as it influences the relationship between variables in epidemiological studies.
Step-by-step explanation:
The factor that is particularly significant to designing a program for the geriatric population in epidemiology, as it alters the outcome, is known as the moderating factor. A moderating factor influences the strength or direction of a relationship between two variables such as a risk factor and a health outcome. For instance, the presence of comorbidities in the elderly could moderate how a health program impacts their overall health - certain programs may be more or less effective depending on existing health conditions. Similarly, access to resources, level of social support, and cultural beliefs could also act as moderating factors. Understanding and accounting for these moderating factors is crucial in tailoring the health program to be as effective as possible for the target population.