Final answer:
Carbon is the least likely to form an ion because, as a nonmetal, it tends to share electrons rather than form ions, unlike sodium, oxygen, and chlorine which commonly form cations and anions.
Step-by-step explanation:
The atom least likely to form an ion among the options provided is carbon (a). This is based on the general properties of the elements and their common ionic states. Nonmetals like carbon tend to share electrons rather than form ions. In contrast, elements like sodium (d), which is a metal, readily lose their valence electron to form a cation (Na+), as shown by its low electronegativity and its desire to achieve a full octet. Similarly, nonmetals like oxygen and chlorine, due to their high electronegativity, are more likely to gain electrons to form anions (O2- and Cl-, respectively).