It is a primary key composed of two or more columns that together guarantee uniqueness.

• A. It uniquely identifies a row using a single attribute.
• B. It ensures all columns are unique.
• C. It is a primary key composed of two or more columns that together guarantee uniqueness.
• D. It allows null values in the key.

Answer: (C)

A composite primary key uses two or more columns to uniquely identify each row. When those column values are combined, they create a unique identifier for every record, even if each individual column alone isn’t unique. This is handy when no single attribute uniquely identifies a row, but the pair (or trio, etc.) does. For example, an enrollment table might use StudentID and CourseID together as the key—the combination is unique for each enrollment. Primary keys also cannot contain nulls, so every part of the composite key must have a value. The other descriptions describe either a simple key (one attribute), a situation where every column is unique (not required for a primary key), or allowing nulls (not allowed for keys).