The correct option is
D extent
Option A is the correct answer because going to a certain extent means I go to
the particular degree to which something is or is believed to be the case, here the Third World War, hence it is correct. Option B is incorrect because “to take sides” means to support one particular cause among the two, but here the scientists just signify the consequences of water loss in general, and not a part of any debate or argument.
Option C is incorrect because “to go to the limit” which means a point or level beyond which something does not or may not extend or pass, hence incorrect.
Option D is incorrect because “situation” means a set of circumstances in which one finds oneself; a state of affairs, hence it is not the correct word here.