Final answer:
The correct statement is 'Key is the super key,' indicating that while every key is a super key due to its ability to uniquely identify tuples, not all super keys are keys because they may not be minimal. Relations can have multiple candidate keys, each of which is a super key.
Step-by-step explanation:
The statement that is TRUE among the options provided is c. Key is the super key. In database terminology, a super key is a set of one or more attributes (columns) that, taken collectively, allows us to identify uniquely a tuple (row) in a relation. A key, sometimes referred to as a candidate key, is a special kind of super key, specifically the minimal set of attributes which can uniquely identify a tuple in a table.
Therefore, while every key is a form of super key, not all super keys are keys because they might not be minimal; some super keys can have additional attributes that are not necessary for unique identification. Moreover, a relation can have more than one candidate key but each of these candidate keys will qualify as a super key. The option that suggests a relation can have only one key is incorrect, as a relation can have multiple candidate keys.