I believe the answer is C because the first two are mechanical hazards and C mentions plug which is electrical.
Answer : The number of moles of oxygen needed are, 1.5 moles.
Explanation :
The balanced chemical reaction will be:

Now we have to calculate the moles of oxygen.
From the balanced chemical reaction we conclude that,
As, 6 moles of water vapor produces from 5 moles of oxygen
So, 1.80 moles of water vapor produces from
moles of oxygen
Therefore, the number of moles of oxygen needed are, 1.5 moles.
When the little fish die they sink to the bottom, and after time the skeletons build up on top of each other.