The correct option is A x2+y2+gx+fy=0
Let midpoint of the chord be P(h,k). So its equation is given by,
T=S1⇒hx+ky+g(x+h)+f(x+k)+c=h2+k2+2gh+2fk+c
But it is given that it passes through (0,0)
⇒h2+k2+gh+ky=0
Hence locus of P(h,k) is given by, x2+y2+gx+fy=0