The correct answer is $\boxed{0}$.
The curl of the gradient of a scalar field is always zero. This can be shown by using the vector identity $\nabla \times \nabla f = 0$, which is true for any scalar field $f$.
The gradient of a scalar field is a vector field that points in the direction of the greatest rate of change of the scalar field, and its magnitude is equal to the magnitude of the rate of change of the scalar field. The curl of a vector field is a vector field that describes the circulation of the vector field around a point.
If we take the curl of the gradient of a scalar field, we are essentially taking the curl of a vector field that points in the direction of the greatest rate of change of the scalar field. However, the curl of a vector field that points in a particular direction is always zero. This is because the curl of a vector field is a measure of the circulation of the vector field around a point, and a vector field that points in a particular direction does not circulate around a point.
Therefore, the curl of the gradient of a scalar field is always zero.