The correct option is C Atrio-ventricular node (AVN)
The cardiac impulse is initiated in the sino-atrial node also known as natural pacemaker. It then travels to the atrio-ventricular node. From there through the Purkinje fibres to the ventricles. The atrio-ventricular node regulates the rhythm of the heart rate mainly. The delay in conduction of impulse in this node changes the duration of contraction between the atria and the ventricles. Hence AV node is called as the pace-setter.