Since an element’s oxide is acidic in nature, A will be a non-metal because the acidity of non-metal oxides is usually measured in terms of the acidic solutions generated when they react with water.
Non-metallic oxides are formed by the reaction of non-metals with oxygen. They react with a base to give salts and water. Let us consider the Calcium hydroxide (base) reaction with carbon dioxide (non-metallic oxide) to produce salt and water.
Ca(OH)2 + CO2 → CaCO3 + H2O
Other examples of non-metallic oxides – CO, NO2, Cl2O, XeO4