The contraction of heart is known as:-
A. Systole
B. Diastole
C. SA node
D. None of the above.

Hint:
Cardiac cycle is a term used for repeating patterns of contraction and relaxation of heart.
At the beginning all the four chambers of the heart are in a relaxed state. Blood from pulmonary veins and vena cava fills the Left atria and right atria respectively. When a Sino atrial node generates action potential the atria go in a contracted stage.
The phase of contraction is called SYSTOLE and the phase of relaxation is called diastole. During contraction the atrio – ventricular valves get close to prevent the backward flow of blood. Semilunar valves also get closed.

Complete answer:
A. Systole- during the cardiac cycle the phase of contraction is called systole. The atria contracts to fill blood in the ventricles and then ventricles contract to give blood for further oxygenation. Hence, option is correct
B. Diastole:-during cardiac cycle the phase of relaxation is called diastole. When atria pressure decreases after giving blood to ventricles they get relaxed. Thereby after giving blood by ventricles , ventricles also get relaxed. hence these option is incorrect
C. SA node:-a patch of the tissue called Sino-atrial node is present in the upper right corner of the right atrium Hence these option is incorrect
D. None of the above:-A option is correct hence these option is incorrect

Hence, option A is correct.

Note:
The action generated by the SAN is conducted to the ventricular side by the atrio-ventricular node and AV bundle from where the bundle of his transmit it through the entire ventricular musculature this causes contraction of the ventricular muscle known as ventricular systole. The atria now undergo relaxation called as atria diastole which coincides with ventricular systole.