Last time I heard this question, the answer was the South China Sea. I've just googled "world's largest sea", and the first six hits give three different answers. I do have to admit though that four of those six websites give the answer that was required for this MQL question.
Not least because their descriptions are very similar, I suspect that the four sites that agree are all ultimately using the same source; but annoyingly, they don't seem to say what that source is.
The Times Concise Atlas of the World (11th edition, reprinted with changes 2011) shows the Philippine Sea as a tiny area, a mere fraction of what many other sources give that name to. Wikipedia, surprisingly, doesn't appear to have a list of the world's largest seas; but on its Philippine Sea page, the word "largest" doesn't appear.
My latest copy of Guinness World Records (it's 2002 – I'm not such an anorak as you think) says that the Pacific is the world's largest ocean, but it doesn't name the largest sea.
The majority consensus now seems to be that the world's ten largest seas are:
1, The Philippine Sea; 2, The Coral Sea; 3, The Arabian Sea; 4, The South China Sea; 5, The Weddel Sea; 6, The Caribbean Sea; 7, The Mediterranean Sea; 8, The Tasman Sea; 9, The Bering Sea; 10. The Bay of Bengal.
The other two candidates are the South China Sea (given by Top 5 of Anything) and the Mediterranean Sea (given by ThoughtCo). Neither of these sources even mentions the Philippine Sea; and there, it seems, is the rub. It seems to me that at some time in the last six years, someone has decided that a large chunk of what used to be the Pacific Ocean now belongs to the Philippine Sea. But who it was that made this decision, or what their authority is, I have no idea.
You'd like to think that the person who set the question knew – or at least knew that it had changed. Assuming they did, it would've been fairer to at least mention that it had changed.
© Macclesfield Quiz League 2017