**Core Concept**
The second heart sound (S2) is a crucial component of the cardiac cycle, produced by the closure of the aortic and pulmonary valves. The duration of S2 is influenced by the timing and coordination of these valve closures.

**Why the Correct Answer is Right**
The duration of S2 is approximately 300 milliseconds (ms), which is the time it takes for the aortic valve to close after the pulmonary valve. This brief period is essential for the proper transition from systole to diastole, allowing for the efficient exchange of blood between the heart and the lungs. The aortic valve's closure is facilitated by the elastic recoil of the aorta and the pulmonary valve's closure is facilitated by the decrease in intrathoracic pressure.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as the duration of S1 is typically longer than S2, approximately 250-300 ms.
**Option B:** This option is incorrect as the duration of S2 is not directly related to the duration of the cardiac cycle.
**Option C:** This option is incorrect as the duration of S2 can vary depending on factors such as heart rate and valve function.

**Clinical Pearl / High-Yield Fact**
In clinical practice, an abnormal duration of S2 can be an indicator of underlying cardiac conditions, such as aortic regurgitation or pulmonary stenosis.

**Correct Answer:** D. 300 milliseconds.