Hi!
The selection in this case would be natural selection.
Natural selection is a process that involves organisms with favorable characteristics having a higher chance of reproductive success, and passing on their traits onto the offspring. This process, as opposed to artificial selection, does not involve human intervention and is a natural phenomenon.
In our case, whales with maximum length (the favorable trait) are mating on accord of their own, without external factors governing the process; hence, the selection is natural.
Hope it helps!