Let the superscript $\text{T}$ represent the transpose operation. Consider the function $f(x)=\frac{1}{2}x^TQx-r^Tx$, where $x$ and $r$ are $n \times 1$ vectors and $\text{Q}$ is a symmetric $n \times n$ matrix. The stationary point of $f(x)$ is

1. $Q^{T}r$
1. $Q^{-1}r$
1. $\frac{r}{r^{T}r}$
1. $r$