a- they get o2 from the food they consume
b-they do not brethe o2 because they do not use o2
c-they come to the surface periodically to brethe air
d-from o2 desolves in the water
e-H2O passing trough the gill is chemically converted to o2(and of course H2)
The two main sources of oxygen in the water for fish is from absorption from the atmosphere. This is inhanced in rough water that helps aeriate the water. The other source is through water plants and algae. They both produce oxygen gas during photosynthesis and much of this is absorbed in the water.