**Core Concept**
Monitoring apnea in preterm babies is crucial to prevent hypoxia and other complications. The best method involves observing the baby's chest wall movement, which is a reliable indicator of breathing efforts.

**Why the Correct Answer is Right**
The correct answer is the use of a _**nasal thermistor**_ to monitor nasal airflow. A nasal thermistor is a small probe placed under the nasal strip to detect the change in temperature when the baby inhales or exhales. This is a reliable method to detect apnea in preterm babies, as it directly measures the airflow. The nasal thermistor is particularly useful in non-ventilated babies, as it can detect even brief apneic episodes.

**Why Each Wrong Option is Incorrect**
**Option A:** Chest X-ray is not a real-time method to monitor breathing and detect apnea. While it can show lung abnormalities, it is not useful for continuous monitoring.

**Option B:** Heart rate monitoring is an indirect indicator of breathing, but it may not detect apnea if the baby's heart rate is stable. It is often used in conjunction with other methods, such as nasal thermistor.

**Option C:** Pulse oximetry measures oxygen saturation, not breathing itself. It can detect hypoxia, but not apnea.

**Option D:** Capnography measures carbon dioxide levels in exhaled breath, which can indicate ventilatory status. However, it requires a breathing circuit and is not suitable for non-ventilated babies.

**Clinical Pearl / High-Yield Fact**
Preterm babies are at risk of apnea due to immaturity of the respiratory center and the brainstem. Regular monitoring and early detection of apnea can prevent complications and improve outcomes.

**Correct Answer:** B. Heart rate monitoring is an indirect indicator of breathing, but it may not detect apnea if the baby's heart rate is stable. It is often used in conjunction with other methods, such as nasal thermistor.