Answer: Option (a) is the correct answer.
Explanation:
Atomic number of magnesium is 12 and electrons are distributed in its shell as 2, 8, 2. So, in order to attain stability magnesium easily loses two electrons.
Therefore, there will be decrease in number of electrons as a result, a positive charge will occur and thus, a cation will be formed.
Thus, we can conclude that an atom of magnesium has lost two electrons. It is known as a cation.