 Solution

Take any differentiable curve $r(s)$ on the level surface. The chain rule givesIts tangent vector $r^{\prime }(s)$ can be any tangent direction at a regular point. Thus a nonzero gradient is orthogonal to the entire tangent plane, giving the unit normal $n=\pm \nabla f/|\nabla f|$. Nonvanishing is essential: at a critical point, the zero gradient alone does not determine a normal or guarantee a smooth level surface.