c
\(\vec F\,\, = \,\, - \,\hat i\,\frac{{\partial U}}{{\partial x}}\,\, - \,\,\hat j\,\frac{{\partial U}}{{\partial y}}\,\, - \,\,\hat k\,\frac{{\partial U}}{{\partial z}}\)

\(\frac{{\partial U}}{{\partial x}}\,\, = \,\,2kx\,,\,\,\,\frac{{\partial U}}{{\partial y}}\,\, = \,\,2ky\,,\,\,\,\frac{{\partial U}}{{\partial z}}\,\, = \,\,2kz\)

\(\vec F\, = \,\, - k(2x\hat i\,\, + \,\,2y\hat j\,\, + \,\,2z\hat k)\,\,\,\,\therefore \,\,\,\,\,\vec F\,\, = \,\, - \,2k(x\hat i\,\, + \,\,y\hat j\,\, + \,\,z\hat k)\)