Nucleic acids are the polymers of nucleotides and they are joined together by a phosphodiester linkage. Nucleotides are the structural units of nucleic acids and are composed of a nitrogenous base, a five carbon sugar and a phosphate group. Thus, option (ii) is correct.

Option (i) is incorrect because nucleosides are composed of a nitrogenous base and a five carbon sugar. They don’t have the phosphate groups. So, they don’t have a phosphodiester linkage.

Options (iii) and (iv) are incorrect because nucleic acids are the polymers of nucleotides and not just nitrogenous bases or pentose sugars.