What can cause alterations in the DNA of a gene?

The correct answer is mutation, as mutations are the direct alterations in the DNA sequence of a gene. These alterations can occur through various mechanisms, such as mistakes during DNA replication, environmental factors, or the influence of specific chemicals. Mutations can range from a single nucleotide change to more extensive changes that affect larger segments of DNA. Such variations can lead to the creation of new alleles, which can subsequently influence an organism's traits if they affect coding regions of genes or regulatory sequences.

Natural selection, gene flow, and genetic drift are important concepts in evolutionary biology, but they describe processes that influence the frequency of existing alleles in a population rather than directly altering the DNA sequence of genes. Natural selection acts on variations that already exist within a population, gene flow introduces new genetic material from one population to another, and genetic drift refers to random changes in allele frequencies, particularly in small populations. While these processes can impact genetic diversity and evolutionary trajectories, they do not cause direct mutations in the DNA itself.