The rectangle is actually a square. In fact, we have that the side PQ is equal to 2 other sides of the rectangle (QR and SP), and this is possible only if all the 4 sides of the rectangle are equal: this means it is a square.
SQ is the diagonal of the square:

, but we know that in a square, the diagonal is equal to square root of 2 times the length of the side:

where d is the diagonal and L the length of the side. Since in our square the length of the diagonal is

, the length of the side must be

so, the correct answer is
b) 6.