One of the results of the 2010 gulf of mexico oil spill is the threat to various species in the gulf’s ecosystem; it disturbed and, in some cases, forever altered the of the gulf coast.
a. biodiversity
b. agriculture
c. deforestation
d. desertification

The correct answer is - a. biodiversity.

The oil spill in the Gulf of Mexico caused very big problems and damage to the ecosystem, and with it the biodiversity. Lots of the species (both plants and animals) that lived in the affected area had big losses in numbers and the area at some places was almost totally empty of life organisms. It was  huge disaster that had extremely bad effect on the biodiversity, but not just when the oil spill happened, the effects are long term, and big portion of the damage done to the flora and fauna is irreparable and will never be the same.  

biodiversity

Biodiversity is the variety of life in a specific ecosystem or world. In this case, we are talking about the gulf's ecosystem. Agriculture refers to farming, but on land - not at sea.. Deforestation refers to the cutting down of trees to reduce the forest lands usually for farming or housing development. Desertification is when good, fertile land becomes like a desert. Since the question refers to the gulf which is a body of water, the only possible answer can be biodiversity. The other answers only apply to land.