"which inference can be made from the graphs?
(1)the increase in sewage waste from 1950 to 1970 was due to a decreasing human population.
(2)the decrease in sewage waste shows that the environmental problems associated with land pollution have been solved.
(3)sewage waste is a good source of nutrients for most fish.
(4)increases in sewage waste were responsible for decreasing oxygen levels in the lake."

According to the graph represented above, without any doubts I can say that the  inference which can be made from the graphs is shown in the last option - Increases in sewage waste were responsible for decreasing oxygen levels in the lake. Due to increasing of sewage waste the oxygen in the water has decreased as on the left part of the picture you can see that one line goes up while the other one drops because of previous one. 
Hope it is useful for you! Regards.