Which of the following describes a frameshift mutation?

A frameshift mutation occurs when bases are added or removed from the DNA sequence in a number that is not a multiple of three. This type of mutation changes the way the sequence is read, shifting the "reading frame" of the genetic code. As a result, the grouping of codons—the three-nucleotide sequences that encode amino acids—is altered, leading to potentially significant changes in the protein that is produced.

When a frameshift mutation occurs, all subsequent codons are shifted, which can result in a completely different amino acid sequence. This can lead to premature stop codons, which truncate proteins, or to elongated proteins with different functional properties. It is often detrimental to the organism, as the resulting protein may be nonfunctional or harmful.

In contrast, replacing one base with another describes a point mutation rather than a frameshift, deleting a segment of DNA refers to larger deletions that do not specifically cause a shift in the reading frame, and gene duplication involves an entire gene being copied rather than shifting the base sequence reading. Therefore, the description of a frameshift mutation is accurately captured by the option indicating the addition or removal of bases that alters the reading frame.