Solution:
A chord is a line segment in a circle with both endpoints on the circle. It is a line whose both ends touch the circumference of a circle.
The diameter is a line that touches both ends of a circle and passes through the center of the circle. Hence, a diameter is a special chord that passes through the center of the circle and it is the longest chord in the circle.
Further explanation:
From the image shown, image 1 has a line that is not a chord because its ends do not touch the circle. Image 2 is a chord because its ends touch the circle. That is what differentiates a line from a chord. Its ends must touch the circumference of a circle.
A diameter is also a chord because it meets the condition of a chord. Its ends touch the circle.
What differentiates both is that a chord does not pass through the centre of the circle, while a diameter passes through the centre.
Therefore, a diameter is a chord.