For vector to be perpendicular their dot product should be 0. Cehck each option only C) and D) when undergo dot product with given two vectors gives 0.
Dot product of two vectors xi + yj + zk and ai + bj + ck is given by ax + by + cz.
Now a unit vector is the one that is divided by its magnitude. It means for a given vector when we divide it by its magnitude we get unit vector.
So the vector that will be perpendicular is i+j+k and its magnitude is √3 so answer is option C).