Which of the following is the valid syntax for adding a PRIMARY KEY constraint to an existing

Question:

Which of the following is the valid syntax for adding a PRIMARY KEY constraint to an existing table?
a. ALTER TABLE tablename ADD CONSTRAINT PRIMARY KEY (columnname);
b. ALTER TABLE tablename ADD CONSTRAINT (columnname) PRIMARY KEY constraintname;
c. ALTER TABLE tablename ADD [CONSTRAINT constraintname] PRIMARY KEY;
d. None of the above is valid syntax.