Why is type O negative blood considered the universal red blood cell donor?

Type O-negative blood does not have any antigens (so yes, it lacks all 3). It is called the "universal donor" type because it is compatible with any blood type

(not sure if this is what you need

I think so the answer of this question is Quaternary.

Explanation:

Many proteins are made up of a single polypeptide chain and have only three levels of structure However, some proteins are made up of multiple polypeptide chains, also known as subunits. When these subunits come together, they give the protein its quaternary structure.