The correct option is B
The (x, y) co-ordinates of the masses at O, A, B and C respectively are (x1=0,y1=0),(x2=a,y2=0),(x3=a,y3=a),(x4=0,y4=a).
Therefore, the (x,y) co-ordinates of the centre of mass are xCM=a2andyCM=a2
. The position vector of the centre of mass is a2(i+j)
, which is choice (b)