The ornamental design for a plunger, as shown and described.
FIG. 1 is a lower perspective view of a plunger showing my new design;
FIG. 2 is an exploded perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a left side elevational view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is a top plan view thereof.
The recessed portions of the hand and the inside of the plunger head as represented by the dashed lines of FIG. 4 and FIG. 7 form no part of the claimed design.