The normal blood volume in a human adult is approximately five litres. This means that the total amount of blood circulating throughout the body is about five litres. Option 1, one litre, is too low and does not accurately represent the normal blood volume. Option 2, three litres, is also too low and does not reflect the typical amount of blood in an adult. Option 4, seven litres, is too high and overestimates the normal blood volume. Therefore, the correct answer is option 3, five litres. It is important to note that blood volume can vary depending on factors such as body size, weight, and overall health.