Final answer:
Underweight primarily increases a person's risk for infection and illness due to a weakened immune system and insufficient nutrition.
Step-by-step explanation:
Underweight increases a person's risk for infection and illness. This state of being can heighten vulnerability to various health issues due to a weakened immune system and insufficient nutrients to maintain optimal body function. While being overweight or obese is widely recognized for its significant health risks, such as increasing the likelihood of developing cardiovascular diseases, type 2 diabetes, and certain types of cancer, being underweight can also pose serious health concerns. While undernutrition can increase the risk of infectious disease, it is not commonly associated with chronic conditions such as high blood pressure, atherosclerosis, or the development of cancer. These conditions are more typically linked to the strain of carrying extra weight and the metabolic syndromes associated with obesity. Hence, the correct answer to the student's question would be option (a): infection and illness.