Final answer:
A foreign key does not need to abide by the condition 'no two tuples agree on all attributes of the key', unlike primary keys and unique keys, which ensure uniqueness in a table.
Step-by-step explanation:
The question pertains to the properties of keys in database management systems. The condition 'no two tuples agree on all attributes of the key' is chiefly related to primary keys and unique keys. Both primary and unique keys ensure uniqueness within a database table, meaning that no two rows can have the same value for those key attributes. On the other hand, a foreign key is used to link two tables together rather than enforce uniqueness within its own table. Therefore, the foreign key can have repeated values and does not necessarily abide by the condition 'no two tuples agree on all attributes of the key'.
A primary key is a column or a group of columns in a database table that uniquely identifies each record in the table. It is used to enforce entity integrity and prevent duplicate data.
A foreign key is a field or a group of fields in a table that refers to the primary key of another table. It establishes a relationship between two tables, and it is used to maintain data integrity.
A unique key is a constraint that determines that all values stored in a column or a set of columns must be unique. However, a unique key does not prevent duplicates on the key's condition.
Therefore, the correct answer is C. None of the others, as primary key, foreign key, and unique key all can have tuples that agree on all attributes of the key.