The ornamental design for a sock, as shown and described.
FIG. 1 is a front perspective view of a new design for a sock.
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The dashed lines on claimed surface at the toe of the sock show unclaimed stitching. The dash-dot-dash line shows environmental subject matter. The dashed lines in FIG. 3 encircling the opening of the sock depict a portion of the article and form no part of the claim.