The argentinosaurus (discovered in 1987 – in Argentina, as its name suggests) would appear to have been longer than a blue whale (up to 35 metres, compared to 30 metres); but the blue whale is much heavier – up to 200 tonnes, as against 75 tonnes.
According to the BBC, "Far bigger than any dinosaur, the blue whale is the largest known animal to have ever lived." If the BBC says it, that would normally be good enough for me; but in a quiz, we should maybe specify what we mean by largest. I suggest that it would be better to ask for the "heaviest" creature (or animal) that's ever lived, rather than the largest.
© Haydn Thompson 2023