Which of the following is a common side effect of chemotherapy?

Hair loss is a well-documented side effect of chemotherapy. Chemotherapy works by targeting rapidly dividing cells, a characteristic of cancer cells. However, it also affects other rapidly dividing cells in the body, including those in hair follicles. This disruption can lead to thinning hair or complete hair loss, a condition known as alopecia.

The other options mentioned do not align with common side effects of chemotherapy. Increased energy levels or improved immune response are not typical outcomes; in fact, chemotherapy often leads to fatigue and a weakened immune system due to its extensive impact on the body's cellular functions. Enhanced digestive health is also not supported, as chemotherapy can cause gastrointestinal issues such as nausea, vomiting, and diarrhea, further demonstrating that hair loss is the correct and relevant side effect in this context.