Final answer:
Chlorine (Cl) is the correct answer because it fits into the less reactive column of the halogens in the periodic table, being more reactive than bromine and iodine but less so than fluorine.
Step-by-step explanation:
Based on the trends in the periodic table and given that we want to find an element that could be added to a column of less reactive elements, the correct choice would be element B) Chlorine (Cl). Chlorine is part of the halogen group (group 17), which includes elements like fluorine (F), bromine (Br), and iodine (I). These elements show a decrease in reactivity down the group, with fluorine being the most reactive. Among the options given, sodium (Na) and potassium (K) belong to group 1 (the alkali metals) which are known to be much more reactive than group 17 elements. Furthermore, fluorine (F) is the most reactive halogen, hence it cannot be considered less reactive. Therefore, chlorine, being less reactive than fluorine but more reactive than bromine and iodine, fits into the column of less reactive elements better than the other options.