A

The term 'Nuclear holocaust' refers to the large scale destruction that would occur as a result of nuclear warfare, leading to massive loss of life and irreversible damage to the environment. A thermo-nuclear reaction, on the other hand, is a process that takes place in nuclear weapons but does not specifically refer to the destruction caused by the use of such weapons. The neutron reproduction factor is a concept related to nuclear reactors and is not applicable in this context. Therefore, the correct answer is 'Nuclear holocaust'.