Answer: 0.001 metric tonnes
Step-by-step explanation:
According to Einstein equation, Energy and mass are inter convertible.

E= Energy
m = mass
c= speed of light
Given : Complete combustion of 1.00 kg of coal releases about
of energy.
Given mass: 1 kg
Converting kg to metric ton using the conversion factor:
1 kg=0.001 metric tonnes
Thus 1 kg of coal would also be equal to 0.001 metric tonnes.