Which of the following statements best describes the essential characteristic of an entity's primary key?

a$.$ The main function of the primary key is to guarantee entity integrity and to "describe" the entity.

b$.$ A primary key is the attribute or combination of attributes uniquely identifying an entity's instance in an entity set.

c$.$ The main function of the primary key is to guarantee entity integrity, not to "describe" the entity.

d$.$ It has no intrinsic meaning; values for it can be generated by the DBMS to ensure that unique values are always provided.