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 a section of the basket of indeterminate length and indeterminate width.