5. As shown in Figure 2, in rectangle $A B C D$, $A B=8, B C=4$. If rectangle $A B C D$ is folded along $A C$, then the area of the overlapping part $\triangle A F C$ is ( ).
(A) 12
(B) 10
(C) 8
(D) 6
5.B.

Since $\triangle C D^{\prime} A \cong \triangle A B C$, therefore,
$S_{\triangle A D F}=S_{\triangle C B F}$.
And $\triangle A D^{\prime} F \backsim \triangle C B F$, so,
$\triangle A D^{\prime} F \cong \triangle C B F$.
Let $B F=x$, then $\sqrt{x^{2}+4^{2}}=8-x$.
Solving for $x$ gives $x=3$. Hence $S_{\triangle A F C}=10$.