Answer
Reduction in mass per nucleon after fusion, greatest to least: two H nuclei, two C nuclei, two aluminum nuclei, two iron nuclei.
Work Step by Step
Hydrogen is farthest to the left, and has a lot of mass per nucleon. After a fusion process, the new nucleus will have much less mass per nucleon.
Moving left to right, carbon is next (atomic number 6), and then aluminum. They fuse to form nuclei that are less heavy than iron, so the new nucleus has a reduction in mass per nucleon, with carbon enjoying a greater reduction per nucleon because it starts out with more mass per nucleon than aluminum.
Iron already has the least mass per nucleon, so if it is forced to undergo fusion, it will have no reduction in mass per nucleon.
This is discussed on pages 649-651.
