Write the approximate value of the energy released in the fission of one nucleus of $_{92}^{235}\text{U}$. What is the reason for it?

Approximately 190 MeV energy is released in the fission of one nucleus.

The reason for the energy release is the loss in mass. In a nuclear change due to a radioactive phenomenon (such as decay, fission or fusion), the total sum of masses of product nuclei is always less than the total sum of the masses of reactant nuclei. Thus, there ia a loss of mass, due to which the energy is released.