The autorhythmicity of cardiac pacemaker cells is made possible by the reduced permeability of

Electrical impulses known as action potentials initiate the contraction of the cardiac muscle.
The autorhythmicity of cardiac pacemaker cells is made possible by the reduced permeability of potassium.
 The pacemaker potential is produced by a decrease in membrane permeability to potassium.

“d” is the answer to the question