Final answer:
The WHO's ICF Model for health identifies social support as a key social factor in successful health outcomes.
Step-by-step explanation:
According to the World Health Organization's ICF Model for health, social support is a social factor that enhances a successful outcome. Studies have indicated that individuals with stronger social support have a 50% greater likelihood of survival compared to those with weaker social relationships. This effect of social support on health outcomes is considered comparable to the health impact of quitting smoking and even exceeds several known risk factors for mortality, such as obesity and physical inactivity. Therefore, the answer to the student's question is d) Social support.