Which type of blood group is known as universal donor?

The correct answer is O negative.

Do not confuse with blood group O positive answer, we have provided the link below for better understanding. 

Important Points

• People with O negative blood group are considered universal donors. RBC's which they contain do not have  A, B, or Rh antigens.
• People who have a blood group of AB positive are considered as a universal recipient. Those people who have AB positive blood group have an absence of antibodies to A, B, or Rh in their blood.
• There are four major blood groups: A, B, O, and AB. Blood groups are classification as blood, they are classified on the presence and absence of antibodies and other antigens on the surface of Red blood cells.
• You can see through this link

https://www.redcrossblood.org/donate-blood/blood-types.html#:~:text=AB-, Group%20O%20can%20donate%20red%20blood%20cells%20to%20anybody.,cells%20to%20B's%20and%20AB's.

Additional Information 

• Blood is composed of four components: White blood cells, Red blood cells, plasma, and platelets.
• Red blood cells are red in colour due to the presence of Hemoglobin, which is an iron-containing protein.
• When viruses or bacteria enter the blood, then White blood cells help in fighting infections.
• Blood Plasma is a yellowish liquid component of blood that helps in the transportation of nutrients like minerals, protein, vitamins, sugars, and fats to all parts of the body.

We can see that O- is compatible to donate every other group.