In the given list of elements: 3A, 8B, 12C, and 19E, the alkaline earth metal is represented by 12C.
Alkaline earth metals are found in Group 2 of the periodic table, where the atomic numbers typically include elements like magnesium (Mg, atomic number 12), calcium (Ca, atomic number 20), and others. Given that 12C corresponds to an atomic number of 12, it can be identified as magnesium, which is indeed an alkaline earth metal.