The pH of human blood refers to its level of acidity or alkalinity. It is important for the body to maintain a specific pH range in order to function properly. The correct answer, option 3, states that the pH of human blood is normally between 7.5 and 8.0.

Option 1, 4.5-5.5, suggests a slightly acidic pH. This is incorrect as human blood typically has a pH that is slightly alkaline, not acidic.

Option 2, 5.5-6.5, suggests a pH that is still within the acidic range. This is also incorrect as the pH of human blood is generally higher than 6.5.

Option 4, 8.5-9.0, suggests a higher pH level, which is more alkaline than the pH of human blood. This is incorrect as the pH of human blood is generally lower than 8.5.

Therefore, the correct answer is option 3, which states that the pH of human blood is normally between 7.5 and 8.0, representing a slightly alkaline condition.