Judging from the graph, you can tell that the dissolved oxygen line (the black one) is lowest during July and some parts of September ( or between those two). At the same time, you can see the blue line - for water temparature - is highest; which means that an increase in temperature ( or thermal energy) results in lower dissolved oxygen. The only answer choice that relates to higher temperatures is A, thermal pollution which is just a result of companies or people using natural bodies of water for industrial purposes.
