Answer: Option (a) is the correct answer.
Step-by-step explanation:
When a heavy nucleus of a large element splits into small nuclei then this type of reaction is known as nuclear fission.
For example,

Energy absorbed by the nucleus of large atom causes it to split when it is striked by a neutron or a particle.
Thus, we can conclude that the statement a nucleus spontaneously splits and absorbs energy, best describes nuclear fission.