The design is the visual features of a portion (heel) of the SOCK shown in solid lines in the drawings, whether those features are features of one of shape, configuration, ornament or pattern or are a combination of any of these features. The portions shown in stippled lines do not form part of the design. The drawings show two variants.Drawings of the design are included, in which:Figure 1 is a front view of a first variant of a SOCK, with the SOCK extended in a flat position;Figure 2 is a rear side view of the first variant of a SOCK of Figure 1, with the SOCK extended in a flat position;Figure 3 is a right side view of the first variant of a SOCK of Figure 1, with the SOCK extended in a flat position;Figure 4 is a left side view of the first variant of a SOCK of Figure 1, with the SOCK extended in a flat position;Figure 5 is a right side view of the first variant of a SOCK of Figure 1, with the SOCK bent at a 90º angle from the heel;Figure 6 is a left side view of the first variant of a SOCK of Figure 1, with the SOCK bent at a 90º angle from the heel;Figure 7 is a rear view of the first variant of a SOCK of Figure 1, with the SOCK bent at a 90º angle from the heel;Figure 8 is a bottom view of the first variant of a SOCK of Figure 1, with the SOCK bent at a 90º angle from the heel;Figure 9 is a right side view of the first variant of a SOCK of Figure 1, with the SOCK bent at a 90º angle from the heel and with the SOCK on a foot form;Figure 10 is a front view of a second variant of a SOCK, with the SOCK extended in a flat position;Figure 11 is a bottom plan view of the second variant of a SOCK of Figure 10, with the SOCK extended in a flat position;Figure 12 is a right side view of the second variant of a SOCK of Figure 10, with the SOCK extended in a flat position;Figure 13 is a left side view of the second variant of a SOCK of Figure 10, with the SOCK extended in a flat position;Figure 14 is a right side view of the second variant of a SOCK of Figure 10, with the SOCK bent at a 90º angle from the heel;Figure 15 is a left side view of the second variant of a SOCK of Figure 10, with the SOCK bent at a 90º angle from the heel;Figure 16 is a rear view of the second variant of a SOCK of Figure 10, with the SOCK bent at a 90º angle from the heel;Figure 17 is a bottom plan view of the second variant of a SOCK of Figure 10, with the SOCK bent at a 90º angle from the heel; andFigure 18 is a right side view of the second variant of a SOCK of Figure 10, with the SOCK bent at a 90º angle from the heel.