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.

1) gene flow

- it is described as the transfer of genes from one population to another population so that frequency of gene is increased.

-a zebra migrates to join a different herd of zebras.

2) natural selection

- a mechanism of evolutionary change in which individuals with advantageous traits survive and reproduce whereas the others ( those that can not adapt) eliminate from the population.

-competition for sunlight leads to taller trees.

3) mutation

- permanent change in the nucleotide sequence of dna ( genetic material) that could have harmful or useful consequences.

-the dna of a snake changes to make its venom stronger.

4) genetic drift - it is a random phenomenon ( that occurs by chance such as fire) that results in change in the gene frequency in a small population.    

-a grassfire randomly sweeps through a population of buffalo and kills most of the animals.

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