The eutrophication that has taken place in the gulf of mexico and other locations appears to be due to
(a) excess nutrients from fertilizers
(b) pesticide use along the waterways
(c) heavy metals dumped in the sewage
(d) global warming from human use of fossil fuels
(e) weather alone, because it is only obvious in the summer

(A) excess nutrients from fertilizers.

Explanation:

The eutrophication in the Gulf of Mexico, as in pretty much all other places, is mostly the result of excess nutrients from fertilizers. The agriculture uses lot of fertilizers, and part of the fertilizers are washed through the soil and reach the ground waters, or some just end up into the water bodies directly.

The water bodies are connected, so the nutrients end up in the largest, central water body that takes in all of the surrounding waters. In this case, the rivers manage to bring in the nutrients into the Gulf of Mexico.

The nutrients are mostly nitrogen, phosphorus, and potassium. Their concentrations are became very high because of the fertilizers, much higher than the natural amount of nutrients. This gives good conditions for the development of the algae, and they rapidly spread out and start to dominate the surface of the water. They also suck out the oxygen out of the water. That results in mass dying out of the other marine organisms in the area.