The ornamental design for a sock, as shown and described.
FIG. 1 is a bottom perspective view of a sock showing our new design;
FIG. 2 is a top perspective view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a top view thereof; and,
FIG. 5 is a side view thereof.
The uneven-length broken lines immediately adjacent to the photographic area define the bounds of the claimed design. The even-length broken lines showing the remainder of the sock represent unclaimed environmental subject matter. None of the broken lines form part of the claimed design.