Define compound key
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Is made up of a several pieces of information.
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Uniquely identifies an item in a list.
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Is a combination of each unique key.
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Both (a) and (b).
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Both (c) and (b).
A compound key is made up of multiple pieces of information (attributes) that together uniquely identify an item in a list. Option A correctly states it's made of several pieces. Option B correctly states it uniquely identifies an item. Option C incorrectly says 'combination of each unique key' - compound keys combine attributes, not other keys. Therefore D (both A and B) is correct.
A compound key (also called a composite key) is formed by combining two or more columns/attributes to create a value that uniquely identifies a record in a table — this matches option (a), 'made up of several pieces of information.' It also, by definition, uniquely identifies an item in the list/table, matching option (b). Therefore 'Both (a) and (b)' correctly captures the full definition. Option (c), 'combination of each unique key,' is a looser/less precise phrasing (a compound key isn't necessarily built from other unique keys), so combining it with (b) is not as accurate as combining (a) and (b).