Answer:
A. Decomposing water requires a high activation energy.
Step-by-step explanation:
The leftovers of the division of the molecule of water would be Hydrogen that would be used as fuel, and oxygen which is necessary and its benefical for humans, so there would be no toxical residuals, and the residuals could be thrown to the air, so the most challenging thing that the scientist would face would be the fact that the molecule of water has a really strong bond between the oxygen and the two atoms of hydrogen, so it requires a really high activation energy.