Which one of the following sets of elements has the strongest tendency to form positive ions in gaseous state?

Explanation

The correct answer is Li, Na, K. These elements are alkali metals, located in Group 1 of the periodic table, and are characterized by their single valence electron, which they readily lose to form positive ions (cations) in the gaseous state. This property gives them the strongest tendency compared to the other sets of elements listed. In contrast, the elements in options B, C, and D are less inclined to form positive ions. Be, Mg, Ca are alkaline earth metals and, while they can form positive ions, they have two valence electrons and a higher ionization energy than alkali metals. F, Cl, Br are halogens, which are more likely to gain electrons and form anions. Lastly, O, S, Se are chalcogens, which also tend to gain electrons to achieve a stable electronic configuration.