On the basis of electronegativity, Na and K is most likely to form monatomic ions.
A monatomic ion is an ion that is made of more than one atom and is from an element like calcium consisting of Ca²+ and a polyatomic ion that is Co²/³.
Many elements such as hydrogen, lithium, sodium, potassium, and magnesium consist of positively charged ions.
Other like fluoride, chlorine, and iodine, and hydride consist of negatively charged ions.
Ba and Ca form diatomic ions.
Thus the correct option is Na and Cl which is sodium and potassium.
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