Which option has the correct pair of compound and mixture?

Correct answer: Sodium chloride and air

Direct answer: Option A, sodium chloride and air. A compound is a pure substance formed when elements combine chemically in a fixed ratio; sodium chloride, NaCl, contains sodium and chlorine chemically combined. A mixture contains two or more substances physically combined in variable amounts; air contains mainly nitrogen and oxygen along with smaller amounts of other gases. Thus the requested pair is compound first and mixture second, exactly as in A. B reverses them: air is the mixture and sodium chloride is the compound, so it does not fit the order asked. C gives copper and oxygen as elements, not a clearly identified compound-and-mixture pair; merely listing substances does not make the pair correct. D gives sulfur and carbon, also elements, not the required pair. A formula often suggests a compound, but the formula alone is not the whole rule; chemical combination and fixed composition matter. Memory cue: NaCl is a fixed compound; air is a variable mixture.