The repeating units of DNA are:

The correct answer is Option (3) → Nucleotides

(1) Amino acids — Incorrect. Amino acids are the repeating units of proteins, joined by peptide bonds. DNA is not made of them.

(2) Nucleosides — Incorrect. A nucleoside is only a nitrogenous base joined to a pentose sugar. It has no phosphate group, so it cannot be linked into a chain.

(3) Nucleotides — Correct. A nucleotide has three parts: a nitrogenous base, a pentose sugar (deoxyribose in DNA) and a phosphate group. Many nucleotides are joined one after another by phosphodiester bonds. This forms the polynucleotide chain, which is DNA. So the nucleotide is the repeating unit.

(4) Saccharides — Incorrect. Saccharides are sugars. The sugar is only one part of a nucleotide, so it cannot be called the repeating unit by itself.

NCERT Extract:
Structure of Polynucleotide Chain – “… A nucleotide has three components – a nitrogenous base, a pentose sugar (ribose in case of RNA, and deoxyribose for DNA), and a phosphate group.”
Structure of Polynucleotide Chain – “… Two nucleotides are linked through 3′-5′ phosphodiester linkage to form a dinucleotide. More nucleotides can be joined in such a manner to form a polynucleotide chain.”