Ans: (C)
Hint : Midpoint of the line segment joining \((1,2,3)\) and \((3,4,5)\) is \((2,3,4)\) and the d.r.'s joining them are \((2,2,2) \equiv(1,1,1)\) \(\therefore\) the required equation of the plane is \((x-2)+(y-3)+(z-4)=0 \Rightarrow x+y+z=9\)