The ornamental design for an undermountsinkbasin, substantially as shown and described.
FIG. 1 is a front, raised perspective view of an undermount sink basin according to the invention;
FIG. 2 is a front view of the undermount sink basin according to FIG. 1;
FIG. 3 is a rear view of the undermount sink basin according to FIG. 1;
FIG. 4 is left side view of the undermount sink basin according to FIG. 1;
FIG. 5 is a right side view of the undermount sink basin according to FIG. 1;
FIG. 6 is a top view of the undermount sink basin according to FIG. 1; and,
FIG. 7 is a bottom side view of the undermount sink basin according to FIG. 1.
The broken lines shown in the drawings form no part of the claimed design. The boundary lines shown in the drawings depict an edge of the claimed subject matter. The surface shading illustrated in the drawings is provided merely to highlight the contour of the design and is not part of the claimed design. All surfaces not shown form no part of the claimed design.