I think it's d.
Consider a point in the plane P(x0,y0,z0) then 2x0-y0+z0=9.
The point becomes a vector v=<x0,y0,z0> in relation to an origin O.
Let Q(x,y,z) be another point on the plane. Its vector is u=<x,y,z>.
OP+PQ=OR, so v+PQ=u, PQ=u-v=<x-x0,y-y0,z-z0>.
The normal n to PQ is the normal to the plane and the dot product n.PQ=0:
n.<x-x0,y-y0,z-z0>=0. Let n=<a,b,c>, then a(x-x0)+b(y-y0)+c(z-z0)=0.
That is, ax+by+cz=ax0+by0+cz0. The RHS is a constant and we have the equation of the plane, so:
ax0+by0+cz0=9 and ax+by+cz=2x-y+z, making n=<2,-1,1>.