Which of the following structures is part of the reproductive system?

The testes are a crucial component of the male reproductive system. They are responsible for producing sperm and hormones, particularly testosterone, which plays a key role in male sexual development and reproductive function. Located within the scrotum, the testes are essential not only for gamete production but also for the regulation of several secondary sexual characteristics.

The other structures provided do not belong to the reproductive system. The heart is part of the circulatory system, responsible for pumping blood throughout the body. The lungs are vital for respiration, facilitating the exchange of gases, and belong to the respiratory system. Lastly, the kidneys are crucial for the urinary system, involved in filtering waste from the blood and regulating fluid balance. Each of these organs plays a significant role in their respective systems, while the testes specifically serve reproductive functions, making them the correct answer in this context.