The ornamental design for the hook, as shown and described.
FIG. 1 is a perspective view of a hook, showing my new design;
FIG. 2 is an enlarged, front elevational view of the invention.
FIG. 3 is a rear elevational view of the invention;
FIG. 4 is a left-side elevational view of the invention, the right-side elevational view being a mirror image thereof;
FIG. 5 is a top plan view of the invention; and,
FIG. 6 is a bottom plan view of the invention.
The broken lines showing a person carrying a load on a strap in FIG. 1 are included for the purpose of illustrating the hook in use, and form no part of the claimed design.