There needs to be some explanation of why there is such a big discrepancy or a link to their source since Wikipedia is generally considered the go to source for most quizzes.
I love these profitless arguments
"The International Hydrographic Organization defines the limits of the Mediterranean Sea as follows: Stretching from the Strait of Gibraltar in the west to the entrances to the Dardanelles..."
I think the important word there is "entrances" - Istanbul is, from that point of view, at the exit. Although to my mind it seems strange to exclude the sea of Marmara from the Mediterranean - it seems more obvious to my tiny mind to have Istanbul as the point where the Mediterranean and Black seas meet. But what do I know!