Final answer:
The electron configuration 1s2 2s2 2p6 3s1 is most likely to form Na+ ion.
Step-by-step explanation:
The electron configuration of an atom determines the ion it is likely to form. In this case, the given electron configuration is 1s2 2s2 2p6 3s1. The outermost electron in this configuration is in the 3s orbital, which means it will be lost to form a positive ion.
Among the given options, only Na+ and Mg+ are positive ions. Na+ loses the outermost 3s electron to have a stable configuration of 1s2 2s2 2p6, while Mg+ loses two electrons to have a stable configuration of 1s2 2s2 2p6 3s2.
Therefore, the ion most likely to form in this configuration is Na+.