A, B, C and D are the points of intersection with the coordinate axes of the lines
ax+by=ab and bx+ay=ab then ,
A, B, C, D are concyclic
A, B, C, D form a parallelogram
A, B, C, D form a rhombus
none of these
The coordinates of A, B, C and D are A (b, 0), B (0, a), C (n, 0), D (0, b) respectively.
Clearly , OA⋅OC=OB⋅OD=ab
So, A, B, C, D are concyclic points.