The ornamental design for a basin, as shown and described.
FIG. 1 is a perspective view of a basin showing my new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a left side elevational view thereof; and,
FIG. 5 is a right side elevational view thereof.
The holes for a fixture and a drain and the drain fixture are shown in broken lines for illustrative purposes only and forms no part of the claimed design.
The back and bottom sides have no decorative features and form no part of the claimed design.