Differentiating a bilinear form wrt x

Question

Let f be a function from $R^4$ to $R$ such that $f(x)= x^TAx$ where A is a 4x4 real matrix.

The gradient of f at the point $x_0$ is

1. $2Ax_0$
2. $Ax_0 + A^Tx_0$
3. $2A^Tx_0$
4. $Ax_0$

How does one approach this? I tried to do brute force differentiation but I don't know how to differentiate it.