The ornamental design for a hook, as shown and described.
FIG. 1 is a perspective view of a hook showing my design;
FIG. 2 is a right side elevational view;
FIG. 3 is a left side elevational view;
FIG. 4 is a top plan view;
FIG. 5 is a bottom plan view;
FIG. 6 is a front elevational view; and,
FIG. 7 is a rear elevational view.
The broken lines showing multiple hooks, openings and a portion of the support plate are for the purposes of illustrating environment and form no part of the claimed design.