Answer : (A) Nucleotides

Step 1 — Recall how a nucleic acid is built

A nucleic acid (DNA or RNA) is a long chain in which the repeating unit is the nucleotide.

Step 2 — Distinguish nucleoside from nucleotide

$$\text{Base} + \text{Sugar} \rightarrow \text{Nucleoside} \xrightarrow{H_3PO_4} \text{Nucleotide}$$

Step 3 — See how the chain is formed

Nucleotides are joined to one another through phosphodiester linkages, the phosphate bridging the C-5 of one sugar and the C-3 of the next.

$$\text{Nucleotide} - \text{Nucleotide} - \text{Nucleotide} - \cdots = \text{Nucleic acid}$$

Step 4 — Reject the other options

Nucleosides lack the phosphate, so they cannot form the backbone.

Bases and sugars are only components of a nucleotide, not the repeating unit itself.