The diameter of the circle is the longest chord that passes through the centre of the circle.

Here, O is the centre of the circle and AB is its diameter.

Diameter is the longest chord of the circle. It divides the circle into exactly two halves.

A line segment passing through the center of the circle and having its end position on the circle is called Diameter.
A and B are the endpoints that are lying on the circumference of the circle.

Circles that have the same center but different radii are called concentric circles.

Answer: Option B. ->

${180}^{\circ }$ of a circle is half of a circle (that is half of ${360}^{\circ }={180}^{\circ }$)

The above image is half of a circle. That is ${180}^{\circ }$ of a circle.

The length of the boundary or the distance around a circle is known as the circumference.

We know that the radius of the circle is half of the diameter.
Therefore, if diameter is 18, then radius is half of 18, which is 9 cm.

The diameter is the longest chord that divides the circle into exactly two halves. Each part is called as Semi-Circle.

Radius of the yellow circle is 5 cm.
Blue circle is the third circle from the centre of the archery board. So, its radius is 5 cm + 2 cm + 2 cm = 9 cm.
Since the diameter of a circle is twice its radius, diameter equals 2 $\times$ 9 cm = 18 cm.

The perimeter of a circle is called the circumference.
Semi-circle is half of the circle and its circumference is half of a circle’s circumference.