Which of the following is called pacemaker?
A. AV node
B. Bundle of His
C. SA node
D. Purkinje fibres

Hint: The heart is a vital organ and the pumping action of the heart takes place in a rhythmic manner. This action of pumping blood to different parts of the body is necessary so that blood can reach to different tissues of the body.

Complete answer:
The rhythmic pumping action of the heart takes place in a coordinated manner of contraction and relaxation. The process of the rhythm is determined by the group of cells. These cells are found to be present in the upper right atrium corner and are known as the sinoatrial node or SA node.
The sinoatrial node (SA node) is a group of cells that causes current generation in the form of electrical stimulus. These cells produce an electrical stimulus which travels through the heart via the electrical conduction system. This generated current or electrical stimulus which is known as the action potential, causes contraction of the heart. The SA node continuously works to generate action potential which helps to set the rhythmic condition of the heart. As the SA node is responsible for generation of rhythmic contraction, it is also referred to as the pacemaker of the heart.
The AV node is present at the lower section of the interatrial septum which lies near the opening of the coronary sinus. It functions to conduct the electrical stimulation from the atria to the ventricles. The SA node passes on the impulse to the AV node which activates it. The AV node delays the impulses by about 0.09s and this delay ensures that the atria have ejected their blood into the ventricles after which the ventricles contract.
Bundle of His is made up of a collection of specialized heart muscle cells that transmits the impulse from the AV node to the apex of the fascicular branches. The fascicular branches further lead to the Purkinje fibres. They are specialized fibres composed of electrically stimulated cells that allows the conduction system of the heart to create synchronized contraction. This is very important in maintaining the normal heart rhythm.

Hence, the correct answer is option (C).

Note: The rhythmic contraction is important in the functioning of the heart and it is brought about by generation of electrical stimulation which is known as the action potential. The action potential is generated by the sinoatrial node or SA node. It is thus known as the pacemaker of the heart.