Indian Institute of Foreign Trade 2008 Solved Paper

'disinterested' here means 'not interested' in an unbiased manner. So we have to identify sentences where the context meaning is unbiased.
In sentence 2, the third party mediate will be disinterested
In sentence 4, the author wants to highlight the point that France’s intervention in the dispute was not entirely disinterested. I.e. this is also a correct option choice.
Hence, option D is the correct answer