Final answer:
Magnesium achieves a noble-gas configuration by losing two electrons to form Mg2+, so it gains zero electrons to achieve this state.
Step-by-step explanation:
Magnesium (Mg) achieves a noble-gas electron configuration by losing electrons rather than gaining them. To attain the configuration of the nearest noble gas, which is neon (Ne), magnesium must lose two electrons. Hence, magnesium will form a Mg2+ ion with the electron configuration 1s22s22p6. Therefore, the answer to the question of how many electrons magnesium would have to gain to achieve a noble-gas electron configuration is a. 0 because it does not gain electrons but instead loses two.