In human blood,the oxygen carrier is

Which of the following blood cells in humans is known to be enucleated (lacking a nucleus)?

To whom can $AB$ blood group blood be donated?

$A$ certain road accident patient with unknown blood group needs immediate blood transfusion. His one doctor friend at once offers his blood. What was the blood group of the donor?

$A$ person with blood group-$A$ has:

The ion present in the highest concentration in blood plasma is........