Try to find the relation of diameter to any of the folllowing options.

Try to recall all types of angles.

Think about the definition of radius of circle.

Take out pencil and mark each vertex while counting them.

Think of the definition of diameter of a circle.

Draw a triangle and try to count corner points and line segments.

Try to recall the name of angles.

Try to find out the relation between diameter and longest chord of a circle.

Think about drawing radius in any circle.

Try to relate number of sides to number of angles.