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

Hair loss is a common side effect of chemotherapy primarily because many chemotherapy drugs target rapidly dividing cells, which include not only cancer cells but also normal cells such as those in hair follicles. The impact on hair follicles can lead to temporary hair loss, which is often one of the more visible and distressing side effects for patients undergoing treatment.

In contrast, weight gain is not typically associated with chemotherapy; some patients may experience weight loss or changes in appetite. Improved appetite and increased energy levels are also not common effects of chemotherapy; in fact, many patients may experience fatigue and a decrease in appetite due to the treatments and their impact on overall health. Therefore, hair loss is the most widely recognized and discussed side effect among these options, making it the correct answer.