The correct answer is C: "World War I saw the first use of firebombing in order to decimate cities which resulted in a high civilian casualty rate."
World War I did see the first use of long-range aerial bombing by Imperial German Air Service Zeppelin Gotha bomber planes based in northern Belgium against targets as far as London. Their impact on the outcome of the war was very limited and only explosive, non incendiary, ordnance was used.
Firebombing, this is, the massive used of incendiary aerial bombs to obliterate cities, was broadly adopted by the British Royal Air Force and the United States Army Air Force as of 1943 resulting in an at-the-time unknown destructive phenomenon called fire storm, (with a destructive power greater than that of the atomic bombs used against Japan) which nearly destroyed Hamburg in 1943 (an estimated 300,000 civilian casualties) and Dresden in 1945 (an estimated 80,00 civilian casualties).