Let's calculate the momentum of Fiona, given by the product between its mass and its speed:

Now let's compare it with the momentum of the other animals:
a) the mass of the sea turtle is missing, so we cannot calculate its momentum.
b) the momentum of the dolphin is

c) the momentum of the horse is

d) the momentum of the lion is

And we can see that the correct answer is b), because the momentum of the dolphin is greater than the momentum of Fiona.