All circles are ..................
${\text{A}}{\text{.}}$Similar, not congruent.
${\text{B}}{\text{.}}$Congruent, not similar.
${\text{C}}{\text{.}}$Similar, may be congruent.
${\text{D}}{\text{.}}$Congruent, may be similar.

Hint: Before exercising such questions, we need to know the theory content of circles. It will help us in finding out the properties of circles.

Complete step-by-step answer:
In question, it is asked that all the circles are?
We all know the properties of circles are as follows: Similarity means the same shape but size may be different. All circles have the same shape. All circles are similar.
By Congruent we mean, same shape and same size. Different circles may have the same or different sizes. All circles may be congruent.
So, option “C” is the correct answer.

Note: A circle is the set of points in a plane that are equidistant from a given point. The distance from the centre is called the radius, and the point is called the centre. Twice the radius is known as the diameter.