Which of the following is a non-specific stress response?

The correct choice identifies infection as a non-specific stress response because it triggers a broad physiological response from the body aimed at managing the threat posed by pathogens. When the body encounters an infection, various non-specific immune responses are activated, such as inflammation, fever, and the release of cytokines. These responses are not targeted to a specific pathogen but are instead designed to enhance general immunity and promote healing across various systems, representing a generalized reaction to stressors.

In contrast, sleep and relaxation represent states of recovery and restoration, responding to stress rather than being stress responses themselves. Chronic pain, while it can certainly be a response to stress, is also a specific condition that affects individuals differently and may provoke a variety of responses depending on the individual's experience. Thus, they do not embody the non-specific stress response framework as well as infection does.