Question

what is radioactive decay?
the time it takes for half of a substance to break down
the process through which unstable elements break down into stable elements
the process that can cause rock layers to erode over time
a tool we can use to find the relative age of fossils

Explanation:

Radioactive decay is the process by which unstable atomic nuclei lose energy by emitting radiation. This matches the description "The process through which unstable elements break down into stable elements". The other options are incorrect: "The time it takes for half of a substance to break down" refers to half - life, "The process that can cause rock layers to erode over time" is related to geological erosion processes (not radioactive decay), and "A tool we can use to find the relative age of fossils" is about relative dating methods (not the definition of radioactive decay itself).

Answer:

The process through which unstable elements break down into stable elements.