b
\(I = - \left( {\frac{{\partial V}}{{\partial x}}\hat i + \frac{{\partial V}}{{\partial y}}\hat j + \frac{{\partial V}}{{\partial z}}\hat k} \right)\)
\( = - \left( {3\hat i + 4\hat j + 12\hat k} \right)\) [ \(V = (3x + 4y + 12z)\)]
\(|I|\, = \sqrt {{3^2} + {4^2} + {{12}^2}} = 13\,Nk{g^{ - 1}}\).