\({\left( {2x} \right)^2} + {\left( {2y} \right)^2} = {\ell ^2} \Rightarrow {x^2} + {y^2} = \frac{{{\ell ^2}}}{4}\)
So, locus of \((x,y)\) is a circle with radius \(\frac{\ell }{2}\).
As center of mass will always go downwards. So option \((a)\) is correct.