Ans: D
Note that the line of intersection of planes $ABC$ and $ACD$ is $AC$. Since $BF \perp AC$ and $DF \perp AC$, then the angle between the plane $ABC$ and the plane $ACD$ is $\angle BFD$.
Note that the line of intersection of planes $ABC$ and $ACD$ is $AC$. Since $BF \perp AC$ and $DF \perp AC$, then the angle between the plane $ABC$ and the plane $ACD$ is $\angle BFD$.
