5. As shown in Figure 1, given rectangle $A B C D, A B=$ $8, B C=6$. If the rectangle is folded so that vertices $A$ and $C$ coincide, then the length of the fold line $P Q$ is ( ).
(A) $\frac{15}{2}$
(B) 7
(C) 8
(D) $\frac{17}{2}$
5. A.

Obviously, $A C$ and $P Q$ bisect each other perpendicularly. Therefore,
$$
\triangle P O C \backsim \triangle A D C \Rightarrow \frac{P O}{A D}=\frac{O C}{D C} \text {, }
$$

we get $P Q=\frac{15}{4} \Rightarrow P Q=\frac{15}{2}$.