The ornamental design for a sock, as shown and described.
FIG. 1 is a perspective view of the sock of this invention;
FIG. 2 is a left side elevation view of the sock of this invention;
FIG. 3 is a plan view of a sole portion of the sock of this invention;
FIG. 4 is a plan view of a top portion of the sock of this invention;
FIG. 5 is a right side elevation view of the sock of this invention;
FIG. 6 is a front elevation view of the sock of this invention; and,
FIG. 7 is a rear elevation view of the sock of this invention.
The broken lines depicting a logo on the surface of the sock form no part of the claimed design. The broken lines depicting a leg are directed towards environment and form no part of the claimed design.