Final answer:
In the M-mode of the cardiac cycle, the beginning and end of diastole are denoted by points D to E. The mitral valve is an atrioventricular valve separating the left atrium from the left ventricle.
Step-by-step explanation:
In the M-mode of the cardiac cycle, the points that denote the beginning and the end of diastole are given by the points D to E (option C). During diastole, the heart relaxes and fills with blood. The mitral valve, also known as the bicuspid valve, is an atrioventricular valve separating the left atrium from the left ventricle.