The diameter divides a circle into two equal parts.

Circumference is the length of the boundary of a circle. A part of it is called an arc.

A and B are two points on the circle. The line joining A and B is called chord.

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

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

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

The distance between the center of a circle and any point on the circle is the radius.

Answer: Option A. ->

${90}^{\circ }$ of a circle is quarter of a circle (that is $\frac{1}{4}$ of ${360}^{\circ }={90}^{\circ }$)

The above image is ${90}^{\circ }$ of a circle.
  

Radius of the yellow circle is 5cm
Black circle is the 4th circle from the center. So, its radius is 5 + 2 + 2 + 2 = 11cm
Radius = 11cm. Therefore, diameter is $2\times 11cm$ = 22cm

We know that diameter is twice the radius.
If radius of the pizza is 18cm, then diameter is 2 x 18cm.

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