The pH of human blood is 7.4. pH is a measure of the acidity or alkalinity of a substance. It is measured on a scale of 0 to 14, with 7 being neutral. A pH value below 7 indicates acidity, and a pH value above 7 indicates alkalinity.

Option 1, a pH of 7.2, would suggest slightly acidic blood which is not accurate. Option 2, a pH of 7.8, would indicate mildly alkaline blood which is also incorrect. Option 3, a pH of 6.6, would indicate acidic blood which is not true for a healthy human.

Therefore, option 4, a pH of 7.4, is the correct answer as it represents the slightly alkaline nature of human blood. It is important for the pH of blood to be within a narrow range for proper functioning of the body`s cells and organs.