This reminds me a little of what remains my most upvoted answer on AskHistorians, which asked a somewhat related question about categorisation and perspective in how we think about the Second World War. This, like that question, is fundamentally not a question about history, but a question about how we approach, teach and learn about history. In fact, this is the kind of question that I really like, because it demonstrates a great truth about the nature of historical enquiry - there is no such thing as objective, simple fact. Even something as basic as 'when did the Second World War start' admits multiple possible answers.

How we arrive at our answer depends not on a series of primary sources through which we discover a singular 'truth', but how we approach the conflict conceptually. For instance, in my own field, an age-old question is whether the Second World War actually started in Spain in July 1936. Here, we see what seems to be the beginnings of a grand confrontation between fascism and anti-fascism, the same ideological battlelines that would come to define the war in Europe. It saw the nascent alliance between Italy and Germany as 'fascist' powers solidify as each chose to intervene , confronted by their main ideological enemy, the Soviet Union. The continuities between the Spanish Civil War and the Second World War are striking - the transnational volunteers who fought in Spain spearheaded partisan movements on their return (most famously, Joseph Tito), and exiled Spanish Republicans formed the backbone of the Resistance in southern France (and drove the first Free French tanks to reach Paris in 1944). For these men and women, the continuities were clear: their anti-fascist war in Spain had become an anti-fascist war in Europe.

Equally, however, someone looking at the conflict from a non-European perspective would perhaps be more inclined to view the important continuities as starting from 1937. If nothing else, the Spanish Civil War ended months before the Second World War broke out, and in the meantime the USSR abandoned its policy of building an alliance to confront European fascism directly, aligning itself temporarily with Germany - a significant spanner in the anti-fascist continuity works. In China, however, the continuities are much more straightforward - the Sino-Japanese War was subsumed into the Second World War in 1941, and finished only with the eventual Japanese surrender in the Pacific. In this sense, the Chinese could meaningfully claim to have been fighting the Axis for longer than anyone else.

However, Spain and China were not, by definition, world wars. Each saw the participation of many different nationalities in one way or another, but were not conflicts with global scope. If that's our definition, neither Spain nor China makes sense as a starting point - each was a regional conflict, perhaps with global implications, but not scale. Here, September 1939 starts to make more sense again - Britain and France each controlled very large empires, and their declaration of war on Germany meant that there were now belligerent territories on every continent. However, in practice the fighting was mostly limited to Europe, North Africa and the seas around them - Germany itself did not have a global reach, and beyond occasional naval battles, could not fight on a global scale (even compared to the First World War). It was not until December 1941 that the fighting itself took on a truly global character with Japan and the USA entering the war. In other words, prior to 1941 we had a general European war, and an East Asian war, but not yet a World War - the USA (and arguably the USSR) were the only countries with the resources to fight an intercontinental war, and until they were involved, the term 'World War' was meaningless.

So, depending on where we start from, and how we conceptualise a world war, each of the answers you give and more are possible, and there is no meaningful historical consensus to 'settle on'. Just yesterday, actually, I was having a conversation with an esteemed colleague where we facetiously tried to make the case for even wilder starting points for the Second World War (1933, 1931, 1917, 1914, 1871... I got to the Battle of Vienna in 1683 before they rudely trumped everything with The First Battle of Megiddo). For most (well, some) of the dates we were tossing around, some form of intellectual argument for significant continuities might be made, and the argument would be accepted by our peers, or at least not rejected out of hand for not being in September 1939.