The correct answer is: C. both (a) and (b)
A compound key is a key that is made up of two or more attributes. It is used to uniquely identify a row in a table. For example, in a table of employees, the compound key might be the employee’s name and ID number.
Option A is correct because a compound key is made up of several pieces of information. Option B is correct because a compound key uniquely identifies an item in a list. Option C is the correct answer because it includes both Option A and Option B. Option D is incorrect because a compound key is not a combination of each unique key. Option E is incorrect because the answer is Option C.