Which of these best describes why a metal has the specific physical property of malleability?
a) the metal atoms are loosely packed together.
b) the positive cations are attracted to the negative anions.
c) the ionic bond it forms with a nonmetal stretches the atoms apart.
d) the free electrons enable the atoms to move and roll over each other.

I believe the answer would be D. Metallic bonding forms a “sea” of free electrons. These free electrons are able to move around much more freely than electrons in a regular lattice formation. This ability of the electrons to move around easily means that metal can be hammered into sheets or pulled into wires and still be stable. So the free electrons allow the metal to have the property if malleability.

They are all full and don't need any more electrons in there outer shell.