Final answer:
A Nuclear agent is responsible for the highly destructive explosions with extreme heat and blast impact described in the question. These weapons can annihilate entire cities and produce significant long-term radioactive fallout.
Step-by-step explanation:
The CBRNE agent that is accompanied by a highly destructive explosion, which instantly devastates people and buildings due to extreme heat and impact of the blast, is a Nuclear agent. This kind of explosive device relies on a nuclear reaction to release massive amounts of energy in the form of an explosion. Nuclear weapons are the most destructive weapons created, with the potential to release energy equivalent to thousands of tons of TNT through the process of nuclear fission or fusion. The devastating effects of a nuclear detonation include not only the immediate blast and intense heat but also the ensuing radioactive fallout, which can have long-term health effects on survivors and the environment.
A notable example of the destructive power of nuclear weapons was witnessed during the atomic bombings of Hiroshima and Nagasaki in 1945. Nuclear weapons, such as the ones used in those bombings or the more advanced thermonuclear weapons, have the capability to annihilate entire cities and pose a significant risk of mass casualties. Moreover, radiation-enhanced nuclear weapons, or neutron bombs, can cause significant radiation damage with a smaller blast yield, designed to incapacitate enemy forces while minimizing the destruction to surrounding structures.