Which blood group is known as universal donor ?

The correct answer is Option (3) – O^-

Universal donors are those with an O negative blood type. O negative blood can be used in transfusions for any blood type. Type O is routinely in short supply and in high demand by hospitals – both because it is the most common blood type and because type O negative blood is the universal blood type needed for emergency transfusions and for immune deficient infants.  Types O negative and O positive are in high demand. Only 7% of the population are O negative. However, the need for O negative blood is the highest because it is used most often during emergencies. The need for O+ is high because it is the most frequently occurring blood type (37% of the population).

The universal red cell donor has Type O negative blood.

The universal plasma donor has Type AB blood.