The ornamental design for a basket, as shown and described.
FIG. 1 is a perspective view of a basket showing our new design;
FIG. 2 is a top view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a front view thereof, the back view being identical; and,
FIG. 5 is a right side view thereof, the left side view being identical.
The broken lines shown in the drawings are included for purposes of illustrating portions of or boundaries of the basket and form no part of the claimed design.