Final answer:
Alpha decay reduces the mass of the daughter element, as the parent nucleus emits an alpha particle, resulting in a decrease in atomic number by two and mass number by four, thus enhancing stability.
Step-by-step explanation:
The type of nuclear decay that will reduce the mass of the daughter element, thereby increasing its stability, is alpha decay. In alpha decay, the parent nucleus emits an alpha particle, which consists of two protons and two neutrons. This process results in a decrease in both the atomic number by two and the mass number by four, leading to a lighter and often more stable daughter nucleus. For example, Plutonium-242 (242Pu) decays via alpha emission to become Uranium-238 (238U), shedding an alpha particle in the process.
In contrast, beta decay changes a neutron into a proton, thereby increasing the atomic number of the daughter nucleus without changing the mass number. Gamma decay, on the other hand, involves the emission of gamma rays and does not change the atomic number or the mass number of the nucleus.