Solution

Ammonia (NH3) is a chemical compound while Oxygen and Mercury are elements and Air is a heterogeneous mixture. Hence option C is correct. A chemical element is a species of atoms that have a given number of protons in their nuclei, including the pure substance consisting only of that species. For example, Sodium, Lithium, Oxygen, Hydrogen etc. are elements. A chemical compound is any substance composed of identical molecules consisting of atoms of two or more chemical elements which are chemically linked by chemical bonds. For example, Water (H2O) is a chemical combination of Hydrogen and Oxygen elements in a fixed ratio and is thus called a chemical compound. A mixture is a combination made up of two or more chemical components that are not chemically linked but are physically present together. It can be of two kinds (viz.) Homogeneous mixture - A homogeneous mixture is a mixture in which the composition is uniform throughout the mixture. All solutions would be considered homogeneous. For example a sugar solution has uniform distribution of sugar particles throughout the mixture making it a homogeneous mixture.   Heterogeneous mixture -  A heterogeneous mixture is a mixture with a non-uniform composition. For example, Air which may have different gases present in a non uniform manner.