The correct answer is option 3: lack of blood supply to the heart. A heart attack, also known as myocardial infarction, occurs when there is a blockage or narrowing of the coronary arteries that supply blood to the heart muscle. This blockage is usually caused by a buildup of plaque, which is made up of cholesterol, fat, and other substances. When the blood supply to the heart is significantly reduced or cut off entirely, the heart muscle does not receive enough oxygen and nutrients, leading to damage or death of the heart tissue. Bacteria and viruses can cause infections in the heart, but they are not the primary cause of heart attacks. The option 4, impairment of the heart`s working due to an unknown reason, does not provide a specific cause for a heart attack.