Final answer:
The relational set operator among the given options is a) UNION. It combines the result sets of two or more SELECT statements into one. JOIN is another set operator, but CONCATENATE and MERGE are not set operators.
Step-by-step explanation:
The correct answer to the question is a) UNION. The UNION operator is used in SQL, which is a domain-specific language used in programming and designed for managing data held in a relational database management system. It allows a user to combine the result sets of two or more SELECT statements into a single result set, which includes all the rows that belong to all the SELECT statements in the union. The requirement for the UNION operation is that all the SELECT statements must have the same number of columns in the result sets with similar data types.
While JOIN is also a relational set operator, it is specifically used to combine rows from two or more tables based on a related column between them. CONCATENATE is not a set operator, but a string function used to combine two or more strings. And MERGE is a SQL command that performs update or insert operations on a table based on the results of a join with another table.