Final answer:
The correct transitional expression to clarify the relationship between the ideas in the passage is 'As a result', indicating the cause-and-effect between the British death toll and the disillusionment of writers.
Step-by-step explanation:
The transitional expression that would best clarify the relationship between the ideas in the passage is 'As a result'. The passage describes the enormous British death toll during World War I, and the subsequent disillusionment expressed by British writers with the leadership responsible for the war. Therefore, the transitional expression 'As a result' most accurately links the consequence (writers' disillusionment) with the cause (enormous death toll).