Which of the following describes the cause of a third heart sound (S3)?

The third heart sound (S3) is best understood as resulting from the rapid deceleration of blood as it fills the ventricles during early diastole. When the left ventricle is compliant and fills with blood following the opening of the mitral valve, this rapid influx of blood can create vibrations in the ventricular wall, producing the S3 sound. This sound is often associated with conditions that increase the volume of blood entering the ventricles, such as heart failure or high-output states.

In the context of other options, while atrial contraction (the action that produces the first heart sound S1) does play a role in the overall cardiac cycle, it does not include the third heart sound. Compliance of the left ventricle is important for the heart's function but is not what directly causes the S3 sound. The association with atrial fibrillation is also less relevant; while S3 can sometimes be found in normal physiological states, its presence or absence in specific arrhythmias does not pertain to its cause. Thus, C effectively captures the mechanisms leading to the generation of the third heart sound.