Question

some portions of dna function as a kind of evolutionary clock when comparing two different species because
no mechanism is available for removing mutations that are harmful.
changes to dna always occur quickly and easily.
mutations accumulate at a known rate.
organisms change their anatomy at a constant rate.
portions of the dna have no function.

Explanation:

DNA's evolutionary clock relies on mutations in non-coding or neutral regions accumulating at a relatively constant, measurable rate over time. This allows scientists to estimate the time since two species diverged by comparing the number of such mutations between their genomes. Other options are incorrect: harmful mutations are often removed by natural selection, DNA changes aren't always quick, anatomy doesn't change at a constant rate, and non-functional DNA alone doesn't make it a clock—rate of mutation accumulation is key.

Answer:

mutations accumulate at a known rate.