The ornamental design for a whisk, as shown and described.
FIG. 1 is a perspective view of a whisk incorporating the design of the present invention, showing the top, left side, and front surfaces of the whisk;
FIG. 2 is a top view of the whisk of FIG. 1;
FIG. 3 is a bottom view of the whisk of FIG. 1;
FIG. 4 is a front view of the whisk of FIG. 1;
FIG. 5 is a rear view of the whisk of FIG. 1;
FIG. 6 is a left side view of the whisk of FIG. 1; and,
FIG. 7 is a right side view of the whisk of FIG. 1.