Energy is released during the fission of pu-239 atoms as a result of the

Nuclear fission consists of dividing a heavy nucleus into two or more lighter or smaller nuclei, by means of the bombardment with neutrons to make it unstable. In this process that takes place in the atomic nucleus, neutrons, gamma rays and large amounts of energy are emitted.  

Then, with this division a great release of energy occurs and the emission of two or three neutrons, other particles and gamma rays.  

This means fission is a process in which energy is released by  the separation of the components of the nucleous of the atom.

In other words:

if the thermos has a vacuum innner, then heat through conv currents and conduction are reduced = vacuum flask

b would be the answer