Final answer:
A column with a unique value in a table is known as a key, specifically a primary key which is used to uniquely identify records in a database. The correct answer is a) a key.
Step-by-step explanation:
A column whose value is always unique in a table is called a key. Specifically, this type of key is referred to as a primary key.
A primary key is a special relational database table column (or combination of columns) designated to uniquely identify all table records.
A primary key's main features are that it must contain a unique value for each row of data, and it cannot contain null values. An example of a primary key is the Social Security Number in a table of individuals, as each person has a unique number.
In database management systems, a key is used to uniquely identify each row in a table. It ensures data integrity and allows efficient data retrieval.