Since Oxygen is in group 16, this element has 6 valence electrons. Valence electrons like to become as stable as possible and similar to a noble gas, which has 8 electrons. You need to find an element that will transfer its atoms to the oxygen to make 8 electrons. Looking at the periodic table, Selenium is in group 16 and will have 6 valence electrons (6 + 6 = 12). This can't transfer electrons with Oxygen. Hydrogen is in group 1 with 1 valence electron (6 + 1 = 7). This can't transfer electrons with Oxygen. Strontium is in group 2 with 2 valence electrons (6 + 2 = 8). This will transfer electrons to Oxygen, making it the most stable.
Because the problem says valence electrons are transferred from one atom to the other, it has to be an "ionic" bond (it's not the "covalent" bond because electrons aren't shared with each other). As a matter of fact, an "ionic" bond is formed when a metal combines with a non-metal or a metalloid. Strontium (Sr) is an alkaline earth metal, which is also a metal, and Oxygen (O) is a non-metal. These two atoms will perfectly form (an) "ionic" bond(s). Therefore, option (2) O and Sr is correct. Hope this would help :))
Molecular compounds include a prefix on the first and second elements and a suffix on the second element, whereas Ionic compounds do not have prefixes and only include a suffix on the second element.
