Which of the following is the definition of "foreign key" in a relational database?

A$.$ An attribute that should not be included in a$($n$)$ entity $($table$).$

B$.$ An attribute appears in an entity $($table$)$ can be found in another database

C$.$ A primary key of a table that appears as an attribute in another table to provid logical relationship between the two tables.

D$.$ An attribute in a table that serves as its primary key as well as a unique key table to provide a logical relationship between the two tables.