30. unit vectors in x and z-direction are i and k respectively. which one of the following is the directional derivation of the function f(x, z) = in (r+z) at t

30. Unit vectors in X and Z-direction are i and k respectively. Which one of the following is the directional derivation of the function F(x, z) = In (r+z) at the point P(4, 0), in the direction of ? (a) 22 (b) 1 (c) 1 (d) 22