Final answer:
The mode of decay for Technetium-99m is isomeric transition, which is a form of gamma decay, where the nucleus transitions from a higher to a lower energy state, emitting gamma radiation in the process.
Step-by-step explanation:
The mode of decay for Technetium-99m is isomeric transition (IT), which is often associated with gamma decay. Technetium-99m is a metastable nuclear isomer of technetium-99, meaning it is an excited state of the core isotope that decays to its stable state by emitting a gamma photon. This type of decay involves the nucleus of a radioactive atom transitioning from a higher energy state to a lower energy state, leading to the emission of gamma radiation without a change in the number of protons or neutrons in the nucleus. Therefore, the correct answer is C) Gamma decay.