The acid-base character of an element's oxide reflects the metallic or non-metallic nature of the element, which changes systematically across a period. On the left of period three, sodium and magnesium are metals whose oxides, such as Na_2O and MgO, are basic because they react with water to give alkalis. Toward the middle, aluminium oxide is amphoteric, reacting with both acids and bases. On the right, the non-metals phosphorus, sulphur, and chlorine form acidic oxides such as P_4O_10, SO_3, and Cl_2O_7, which give acids with water. Thus the oxide character changes from basic to acidic across the period. The option acidic to basic reverses the trend. The options of remaining basic or acidic throughout ignore the transition through the amphoteric region. This systematic variation in oxide nature is a standard NCERT periodicity trend. Understanding nature of oxides in this way ties directly into the wider study of classification of elements and periodicity in properties, where the same reasoning recurs across many problems. Examiners frequently test whether a student can connect acidic and basic oxides with the underlying principle rather than merely recalling an isolated fact. Plausibility check: the metallic left forms bases and the non-metallic right forms acids, with amphoteric alumina in between, confirming the basic-to-acidic progression.