The British Admiralty and the War Office met in March 1892 to consider a possible Russian attempt to seize Constantinople and how they would have to act militarily to deal with them. (D) what military action would be necessary in order to deal with such an event
the answer D here, I think no need to write down "in order", it's reductant, like this--- what military action would be necessary to deal with such an event---, it's enough.