Why is nucleic acid a polymer?
Because it consists of monomer building blocks.
A polymer is a large molecule that is built up from multiple smaller building blocks in a repetitive manner.
The building blocks of the nucleic acids DNA and RNA are nucleotides (see image). The nucleotides have a phosphate group, a sugar group and a nitrogenous base (adenine, thymine, guanine, cytosine or uracil).
Many of these building blocks bound together for the nucleic acid i.e. the polymer:
This is an example of a double-stranded nucleic acid = DNA. It can also be a single strand = RNA. Both DNA and RNA are polymers.