The question asks to choose the correct charge for each element from a list containing Na, Cl, Mg, O, Ca, Br and K.
To define the charge of an element, a simple approach is to write the element's electronic configuration and decide whether it would be "easier" for this atom to receive or lost electrons in order to achieve 8 electrons in the valence shell.
Then, let's write the electronic configuration of the given elements:
Na (Z=11): 1s2 2s2 2p6 3s1
Cl (Z =17): 1s2 2s2 2p6 3s2 3p5
Mg (Z=12): 1s2 2s2 2p6 3s2
O (Z=8): 1s2 2s2 2p4
Ca (Z=
