SajjadAhmad
Can a circle be drawn so that its circumference includes all four vertices of quadrilateral Q ?
(1) All four sides of Q are equal in length.
(2) Each of the interior angles of Q measures 90°.
Since we're asked if a circle can be drawn, why not just try? This is a classic Alternative approach.
(1) All four sides of Q are equal in length so it is a square or a more messy rhombus.
We'll try drawing a square inscribed by a circle - yes!
We'll try drawing a rhombus inscribed by a circle - no!
Playing with the figure, we can SEE that this won't work because the diagonals of a rhombus aren't always equal.
Insufficient!
(2) Our quadrilateral is now a rectangle.
Once again, we'll try drawing a rectangle inscribed inside a circle.
No matter how we draw our rectangle, this works!
Therefore (2) is sufficient and we can mark (B) as our answer (even if we don't understand why!)
** Note - (2) is true because we can set the diagonals of the rectangle as the diameter of the circle and the meeting point of the diagonals as its center.