The key declares that an index in one table is related to that in another is called . . . . . . . .

Imagine you have two tables, one for students and another for courses.
Each student can enroll in multiple courses, and each course can have many students.

To connect these tables, we use a special kind of key called a foreign key.
The foreign key in the students table points to the primary key in the courses table.

This way, we can easily find the courses a specific student is enrolled in or the students taking a specific course.

So the answer is Option C: foreign