Answer: ruling out rival hypothesis
Step-by-step explanation:
To rule out rival hypothesis suggests that to exclude the possible explanation for a specific claim. This suggests that before confirming or believing something one has to ensure that other explanation for a specific claim has been disproved to become sure for a specific claim.
According to the given situation, the stress and illness relationship is required to be derived. The increased level of stress and illness is responsible for the decrease in immunity is ruled out by the fact that people due to stress indulge or engage in health damaging behaviors. This is responsible for illness.