Doctors use a stethoscope to hear the sounds produced during each cardiac cycle. The second sound is heard when:

Normal sound of heart that can be heard through the stethoscope is "lupp-dupp". It is the basic sound of heart that can be obtained as a result of each cardiac cycle. The "lupp" is the first sound created due to the closure of mitral and tricuspid valves at the start of systole. The "dupp" is the second sound which is shorter and higher pitched than the first sound, and is produced by closure of the aortic and pulmonary valves, they are commonly referred as semilunar valves. So, the correct answer is option D.