The ornamental design for basket, as shown and described.
1. Basket
1.1: Perspective
1.2: Perspective
1.3: Front
1.4: Right
1.5: Top
1.6: Bottom
The color shown in the representations does not form part of the claimed design; the back view of the design is omitted because it is identical to the front view; the left side view of design is omitted because it is identical to the right-side view.