The element that can form a positive ion out of the choices you provided is magnesium (Mg). Here's why:

* Magnesium is a metal. Metals tend to lose electrons to form positively charged ions (cations). Magnesium, specifically, loses two electrons to achieve a stable electron configuration, forming Mg²⁺.

* Carbon is a nonmetal. Nonmetals tend to gain electrons to form negatively charged ions (anions).

* Oxygen is also a nonmetal and forms negatively charged ions (O²⁻).

* Bromine is a nonmetal and forms negatively charged ions (Br⁻).