7. D.

To maximize the area of region $S$, the two circles with radii $5$ and $7$ should be on opposite sides of the line $l$, and the circles with radii $3$ and $1$ should be internally tangent to one of these circles. Thus, the area of $S$, which consists of all points inside one of the circles, is maximized, as shown in Figure 8. Therefore, the required area is
$$
7^{2} \pi + 5^{2} \pi - 3^{2} \pi = 65 \pi \text{. }
$$
7. In a plane, four circles with radii $1, 3, 5, 7$ are tangent to a line $l$ at a point $A$, and they can be on either side of the line $l$. The region $S$ consists of all points that lie within one of these four circles. What is the maximum area of region $S$? ( ).
(A) $24 \pi$
(B) $32 \pi$
(C) $64 \pi$
(D) $65 \pi$
(E) $84 \pi$