If a child has blood group $O$,which of the following blood groups cannot be possessed by the parents?

In which of the following pairs does erythroblastosis fetalis occur?

$A$ doctor suggested a couple not to have more than one child because:

Which of the following vitamins is essential for the maturation of erythrocytes?

$A$ person with an unknown $ABO$ blood group has suffered a major accident and requires immediate blood transfusion. $A$ friend offers to donate blood and has a valid certificate of their blood group. Which blood group must the friend have to ensure safe donation to any recipient?

The figure shows a human blood cell. Identify it and give its characteristics.