In a nuclear fission 0.1% of mass is converted into energy. The energy released by the fission of 1kg mass is 

As mentioned in the previous response, the energy released by the fission of 1 kg mass can be calculated using Einstein's equation, $E=m{c}^2$, where E is the energy, m is the mass, and c is the speed of light in vacuum.

Given that 0.1% of the mass is converted into energy, we can calculate the energy released as follows:

Energy released = 0.1% of 1 kg mass

$= \frac{0.1}{100}\times 1 kg=0.001kg$

Using the equation $E=m{c}^2$, we have:

$E=0.001kg\times (3\times {10}^{8}m/s{)}^2$

$E=9\times {10}^{13}J=\frac{90\times {10}^{12}}{36\times {10}^5}kWh$

$E=2.5\times {10}^{7}kWh$ (rounded to two significant figures)

Hence the correct answer is $2.5\times {10}^{7}kWh.$