Final answer:
The type of radioactive decay occurring in the provided nuclear equation, which shows thorium absorbing an alpha particle, is alpha capture (d), as opposed to other types such as alpha, beta, or gamma decay.
Step-by-step explanation:
The nuclear equation provided represents alpha capture. During alpha capture, an alpha particle is absorbed by a nucleus. The alpha particle, which is essentially a helium-4 nucleus, consists of two protons and two neutrons. When an alpha particle interacts with another nucleus in this manner, it is being captured rather than being emitted, as seen in alpha decay reactions. The given equation, 20092Th + 42He, implies that thorium is absorbing an alpha particle, which contradicts the process of alpha emission where an alpha particle is released. Therefore, the correct type of radioactive decay in this scenario is alpha capture (d), not alpha (a), beta (b), or gamma (c) decay.