Answer : (C) nucleotides
Step 1 — Recall how a nucleic acid is built
A nucleic acid — DNA or RNA — is a long chain whose repeating unit is the nucleotide.
Step 2 — Distinguish nucleoside from nucleotide
| Term | Components |
|---|
| Nucleoside | Nitrogenous base + Pentose sugar (two parts) |
| Nucleotide | Nitrogenous base + Pentose sugar + Phosphate (three parts) |
$$\text{Base} + \text{Sugar} \rightarrow \text{Nucleoside} \xrightarrow{H_3PO_4} \text{Nucleotide}$$
Step 3 — See why the phosphate is essential
Nucleotides are joined to one another through phosphodiester linkages, the phosphate group bridging C-5 of one sugar and C-3 of the next.
Without the phosphate there would be nothing to link one unit to the next — which is precisely why a nucleoside cannot be the repeating unit.
Step 4 — Reject the other options
Amino acids are the monomers of proteins, joined by peptide bonds.
Pentose sugar is only one component of a nucleotide, not the repeating unit itself.
Step 5 — Note the resulting structure
The alternating sugar and phosphate units form the backbone of the strand, with the nitrogenous bases projecting inwards to pair with those of the opposite strand.
