A key:

Answer: Option C

Option1: A key must always be composed of two or more columns because a composite key is used to uniquely identify a row in a table when a single column key is not sufficient.
Option2: A key can only be one column is incorrect because a key can be composed of multiple columns to uniquely identify a row in a table.
Option3: A key identifies a row is correct. A key is used to uniquely identify a row in a table and ensure that each row is unique.
Option4: A key identifies a column is incorrect. A key is used to identify a row, not a column in a table.
In conclusion, the correct option is Option3: identifies a row. A key is used to uniquely identify a row in a table and ensure data integrity.