Consider a vector $\bar{u}=2 \hat{x}+\hat{y}+2 \hat{z}$, where $\hat{x}, \hat{y}, \hat{z}$ represent unit vectors along the coordinate axes $x, y, z$ respectively. The directional derivative of the function $f(x, y, z)=2 \ln (x y)+\ln (y z)+3 \ln (x z)$ at the point $(x, y, z)=(1,1,1)$ in the direction of $\bar{u}$ is
- $0$
- $\frac{7}{5 \sqrt{2}}$
- $7$
- $21$