Option (iii) is correct because carbon dioxide (compound), iron (element), oxygen (element), and sugar (compound) are all pure substances. The chapter defines pure substances as those that 'consist of the same type of particles' and can be either elements or compounds. Option (i) is incorrect because water is a compound and air is a mixture, not elements. Option (ii) is incorrect because minerals are typically compounds with fixed composition, not mixtures. Option (iv) is incorrect because air and brass are uniform mixtures, not non-uniform.