The ornamental design for a basket, as shown and described.
FIG. 1 is a perspective view of a basket showing my new design.
FIG. 2 is a front view thereof.
FIG. 3 is a rear view thereof.
FIG. 4 is a right side view thereof.
FIG. 5 is a left side view thereof.
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken lines show portions of the design that form no part of the claimed design. The break lines represent sections of the basket of indeterminate length and width.