Answer:
D. The isotope that undergoes radioactive decay to form a more stable isotope.
Explanation:
In radioactivity, isotopes of an unstable atom disintegrates/decays into more stable ones accompanied with the emissions of certain particles e.g alpha, gamma and beta particles.
In this process, the isotope of the atom that undergoes radioactive decay to form a more stable isotope is known as the PARENT ISOTOPE while the more stable isotope that arises from the decay is called the DAUGHTER ISOTOPE.
(238, 92) Uranium → (234,90) Thorium + (4,2) Helium
In the above reaction, Uranium is the parent isotope because it decays into a more stable isotope (Thorium).
