The rate of Heartbeat is determined by
A. SA node
B. AV node
C. Purkinje fibre
D. Papillary muscles

Hint: Heart is the main organ of the circulatory system in most of the animals including human beings. The main function of this organ is to pump the circulatory fluid throughout the body that is blood. Heart rate is defined as the rate at which the entire heart beats. Its range is between 60 to 100 beats per minute.

Complete step by step answer:
The internal structure of heart is mentioned by the following points :
Human heart is four chambered which are two auricles and two ventricles.
The opening and closing of each chamber is regulated and controlled by different types of valves.
A mass of cell called SA node (Sino-atrial node) is present at the upper posterior wall of the right atrium.
It is known as the pacemaker of the heart because it generates electrical impulses.
These electrical impulses spread through the walls of the heart and cause them to contract.
Because of this contraction, blood is moved to ventricles from auricles. This is why we say that the SA node sets the rate and rhythm of the heart.
Another mass called AV node electrically connects the auricles and verticals.
Purkinje fibres send the electrical impulses (generated by SA node) to ventricular walls.
Papillary muscles help in opening and closing of atrioventricular valves.
The cells of the SA node send out electrical signals at a specific rate which determines the rate at which the entire heart beats.
So, the correct answer is option (A) SA node.

Note: SA node stands for SinoAtrial node which indicates that it is present in the atrium. The activity of the SA node that is a pacemaker is observed to detect the heart rate. This process is called ECG (Electrocardiogram).