d. CO (carbon monoxide) is not an acidic oxide.
Acidic oxides are those oxides which either act as acceptors of the hydroxide ions (OH-) and function as a Lewis acid or they produce an acidic solution when added to water. Examples of acidic oxides are P4O10 (Phosphorus pentoxide), P4O6 (Tetraphosphorus hexaoxide), SO3 (Sulfur trioxide), SiO2 (Silicon dioxide) etc.
Neutral oxides do not show either of the basic or acidic properties. Due to this fact, they do not form salts when they react with acids or bases. Examples of neutral oxides include CO (carbon monoxide), NO (Nitric oxide), N2O (Nitrous oxide) etc.
Hence, option d is correct.
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