Which corresponds to the composition of the ion typically formed by magnesium?

The ion composition of Magnesium is 12,10, 2+.
Magnesium is a chemical element with symbol Mg and an atomic number 12, it has 12 protons, and 12 electrons with a chemical configuration of 2:8:2. It requires to loose two electrons to form a stable configuration forming a cation (positively charged ion) with a charge of +2 and  a configuration of 2:8 ( 12 protons and 10 electrons).

which means that the volume increased by 26.4 mL in order to compensate for the decrease in pressure.

Like I said, depends on what your initial volume was, but that's how you think of it.

Hope this helped!