To solve this problem, we need to analyze the properties of nitrogen dioxide (NO$_2$) and sulphur dioxide (SO$_2$).$\\$Let's examine each option:$\\$Option 1: Is a reducing agent.$\\$• Both NO$_2$ and SO$_2$ can act as reducing agents under certain conditions.$\\$Option 2: Is soluble in water.$\\$• Both NO$_2$ and SO$_2$ are soluble in water.$\\$Option 3: Is used as a food preservative.$\\$• SO$_2$ is commonly used as a food preservative due to its antimicrobial properties.$\\$• NO$_2$ is not used as a food preservative.$\\$Option 4: Forms ‘acid rain’.$\\$• Both NO$_2$ and SO$_2$ contribute to acid rain formation.$\\$Therefore, the property that is shown by one of these compounds, but not by the other, is:$\\$Option 3: Is used as a food preservative.$\\$Thus, the correct answer is Option 3.