Which type of white blood cell is least commonly found in human blood?

$A$ person with an unknown blood group ($ABO$ system) has lost a large amount of blood in an accident and needs an immediate blood transfusion. $A$ friend of the person has a valid certificate of their own blood group and is ready to donate blood without delay. What could be the blood group of the donor friend?

Assertion: $Prothrombinase$ enzyme acts as an antiheparin.
Reason: $Heparin$ prevents the coagulation of blood in blood vessels.

Choose the incorrect sentence$(s)$.
$(I)$ $RBCs$ are formed in the white bone marrow in adults.
$(II)$ $RBCs$ have a red colour because of the presence of a protein called haemoglobin.
$(III)$ The nucleus is present in $RBCs$ in most mammals,and they are biconcave in shape.
$(IV)$ $A$ healthy individual has $12-16 \, g$ of haemoglobin in every $100 \, ml$ of blood.

Their count is about $2-3$ percent of the total white blood cell count in the blood.

Who discovered the $Rh$ factor?