What part of a nucleic acid allows it to be used to form a code

The correct answer is nitrogenous bases.

Explanation:

Nucleic acid like DNA and RNA acts as the genetic material of the organism.  The nucleic acid is formed of the monomer units called nucleotide where each nucleotide unit is made up of three molecules: a 5-C sugar (ribose or deoxyribose), a phosphate group, nitrogenous bases (adenine, cytosine, guanine, thymine and uracil (in case of RNA).

It is the arrangement of these nitrogenous bases in the strand of DNA which makes it a genetic material as the ribosome during translation reads these nitrogenous bases from the sequence of mRNA in pair of three bases called "triplet codons". Each codon code for the specific amino acids which make up the protein molecule.  

Thus, nitrogenous base is the correct answer.

sound moves fastest in solids because the molecules are tightly packed and it allows sound to vibrate