The splitting apart of nuclei id known as
fission
fusion
bombardment
fallout

Option A= Fission

Explanation:

Fission:

"It is the process in which nuclei of radioactive elements are bombarded with neutrons and gets converted into the many smaller nuclei"

When the are bombarded with neutrons many smaller nuclei are produced and energy is also released in the form of E = mc². This process is exothermic. The sum of masses of smaller nuclei are not equal to the mass of original nuclei because energy is also released, and missing mass is converted into energy.

Fusion:

"It is the process in which smaller nuclei combine to form the heavier nuclei"

Bombardment:

It is the process in which nucleus is bombarded with the another nuclei or nuclear particles and result may come in the form of fission or fusion.

Fallout:

In this process after the nuclear explosion, radioactive particles in air fall out as a nuclear dust.