The intersection point of $y=1$ and $3x-y=1$ is the point $A=(\frac {2}{3},1)$
The reflection of the same w.r.t to $y-axis$ would be the point $B=(\frac {-2}{3},1)$
Since the triangle $ABC$ is right angled at $B$, the equation of the required line would $3x+2=0$ as it is perpendicular to $y=1$
So, option C is correct