The ornamental design for a sock, as shown and described.
FIG. 1 is a perspective view of a sock showing the claimed design;
FIG. 2 is a back view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a bottom perspective view thereof;
FIG. 5 is a right side view with a detailed portion thereof;
FIG. 6 is a right side view thereof; and,
FIG. 7 is a left side view thereof.
The broken line showing of a sock is for the purpose of depicting portions of a sock that forms no part of the claim. The subject inside of the solid lines depicts the claimed sock portions. Dashed lines in FIG. 5 depict a sectional cut out of the detailed view.