A pure substance can only be: (a).A compound (b).An element (c).An element or a compound (d).A heterogenous mixture

Pure substances are those which involve only one kind of atoms or molecules having constant composition and structure where as in mixture components are not in definite proportion or ratio. With this, you can easily answer it. Complete answer: Pure substances are those substances which are made up of only one kind of particle and have a definite composition (means that a given compound always contains elements in the proportion by weight(mass) and have a fixed or constant structure. E.g. copper, silver, gold, water etc. Glucose contains carbon, hydrogen and oxygen is in a fixed ratio and, thus, like all other pure substances has a fixed composition and thus, is a pure substance. They have characteristics different from the mixtures in which the components are not in any definite ratio and thus have variable composition. The constituents of pure components can be easily separated by physical means i.e. Evaporation, distillation, filtration etc. Pure substances can be either elements or compounds. An element is that which consists of only one type of particle. These particles may be either atoms or molecules. E.g. sodium, copper, silver, hydrogen, oxygen etc. all these contain atoms of one type only.in case of molecules two atoms combine to give their respective molecules. E.g. hydrogen, nitrogen and oxygen gases. Like: H+H→H2 The atom of hydrogen combines with its respective atom to form the hydrogen molecule. On the other hand, in case of compounds, two or more different elements combine resulting in the formation of compounds. E.g. water, ammonia, carbon dioxide etc. Like: C+O2→CO2 Carbon and oxygen two different elements combine to form the compound molecule \i.e. Carbon dioxide. So, thus it is clear that a pure substance can only be an element or a compound. Hence, option(c)is correct. Note: Don’t get confused in the molecules and compounds. Simply, we can say that the molecules are made up of two or more similar atoms of the same element while the compounds are made up of two or more atoms of different elements.