Concept:A chord is any line segment joining two points on a circle. The longest possible chord passes through the centre and is called the diameter.Explanation:A line segment joining any two points on the circumference of a circle is called a chord.The diameter is a special chord that passes through the centre of the circle.It divides the circle into two equal halves and is the longest chord because no other chord can span a greater distance across the circle.Therefore, among the given options, the longest chord of a circle is the diameter.Answer:D. diameter.