The ornamental design for an adjustable multipurpose rack, as shown and described.
FIG. 1 is a perspective view of the top, front and right side of an adjustable multipurpose rack showing my new design, the back and bottom components being identical and the end panels being identical;
FIG. 2 is a perspective view of FIG. 1, the intermediate divider, identical to the end panels, depicted by means of broken lines, being shown for illustrative purposes only and forming no part of the claimed design;
FIG. 3 is a left side elevational view thereof, the right side elevational view being identical thereto;
FIG. 4 is a left side elevational view of an end panel, shown separately for ease of illustration, the right side being a mirror image thereof; and,
FIG. 5 is a perspective view of the top, rear and right side thereof.