The ornamental design for a sock, as shown and described.
FIG. 1 is a front perspective view of a sock showing my new design in an in-use configuration;
FIG. 2 is a bottom rear perspective view thereof;
FIG. 3 is a side view thereof;
FIG. 4 is a second side view thereof;
FIG. 5 is a front view thereof;
FIG. 6 is a rear view thereof;
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view thereof.
The broken lines immediately adjacent to the shaded area represent boundaries of the design and form no part of the claimed design. The broken lines showing the remainder of the sock are for environmental purposes only and form no part of the claimed design.