The ornamental design for a holder for a sock, as shown and described.
FIG. 1 is a front perspective view of a holder for a sock showing my new
design, the broken line showing of the sock and handle is for illustrative
purposes only and forms no part of the claimed design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a front elevational view thereof, the rear is a mirror image;
FIG. 4 is a side elevational view thereof, the opposite side is a mirror
image; and,