Step 1: Peter Agre shared the 2003 Nobel Prize in Chemistry with Roderick MacKinnon.
Step 2: Agre's share of the prize was specifically for discovering aquaporins, the water channel proteins in cell membranes that allow rapid, selective movement of water across the plasma membrane.
Step 3: MacKinnon's share, by contrast, was for structural studies of ion channels, so the other options like nucleosome, pigment or ATPase do not match Agre's discovery.