Final answer:
The immune system can be improved by social support, laughter, and biofeedback, as all these factors contribute positively to immune response and overall immune functioning.
Step-by-step explanation:
The functioning of the immune system can be improved by d. all of the above. This includes a range of practices, such as receiving social support, experiencing laughter, and engaging in biofeedback. Social support has been linked to enhanced immune functioning, especially in individuals under stress.
It seems to positively influence the immune system's response, including better performance on immune functioning measures. Laughter and positive emotions are similarly associated with better immune function, and biofeedback, which involves training to control physiological processes, can also lead to improvements in immune response.