Final answer:
Element 4, with 4 valence electrons, is typically less reactive than the other elements given, which have 1-3 valence electrons. This element, which is halfway to having a full valence shell, will not easily lose or gain electrons, making it the least reactive among the options.
Step-by-step explanation:
The reactivity of an element is greatly influenced by the number of valence electrons it has. Elements with 1-3 valence electrons are typically more reactive because they tend to lose these electrons to achieve a full outer shell, and thus they form positively charged ions. On the other hand, noble gases, with a complete set of valence electrons, are very stable and non-reactive.
In the given question, element 1 (with 1 valence electron) is likely an alkali metal, element 2 (with 2 valence electrons) is likely an alkaline earth metal, element 3 (with 3 valence electrons) is a member of the boron group, and element 4 (with 4 valence electrons) is a member of the carbon group. Given that elements with a completely filled valence shell are least reactive, we look for the one closest to having a full shell. While none of the listed elements has a filled valence shell, element 4, with 4 valence electrons, is halfway to having a full shell and will neither easily lose nor gain electrons. Therefore, it is typically less reactive compared to the other elements listed.
Accordingly, the answer is D. Element 4. This element likely represents carbon (group 14), which is known to be less reactive than alkali metals (group 1) or alkaline earth metals (group 2), as it would need to acquire or lose 4 electrons to reach a full valence shell, which is energetically less favorable.