Answer:
It would be the magnesium ion and the nitrogen ion (N3-).
Step-by-step explanation:
The (not really but sorta) brute force method:
First, count the number of arrows. This is the number of electrons. The number happens to be 10 electrons. All that needs to be done now is to find out which ions have 10 electrons. Be2+ would have beryllium's 4 electrons minus 2 electrons, or 2 electrons in total. beryllium is not the answer. Magnesium has 12 electrons, and so Mg2+ has 10 electrons. Mg2+ is an answer. Nitrogen has 7 electrons, and with 3 more, N3- has 10 electrons. Chlorine has 17 electrons, and with one more it has 18 electrons so it is not chlorine.
Another method (practically the same thing but reworded):
Whatever atom which is depicted has 10 electrons, or the [Ne] electron configuration. Beryllium 2+ is not isoelectronic, so the ion can't be that. Magnesium 2+ has the [Ne] electron config, so does N3-. Cl- doesn't.