Answer : The momentum is same for 50 kg dolphin and 4100 kg elephant.
Explanation :
Momentum : It is defined as the motion of a moving body. Or it is defined as the product of mass of velocity of an object.
Formula of momentum is:

where,
p = momentum
m = mass
v = velocity
First we have to calculate the momentum for 50 kg dolphin swimming at 16.4 m/s.
Given:
m = 50 kg
v = 16.4 m/s
Now put all the given values in the above formula, we get:


Now we have to calculate the momentum for 4100 kg elephant walking 0.20 m/s.
Given:
m = 4100 kg
v = 0.20 m/s
Now put all the given values in the above formula, we get:


From this we conclude that, the momentum is same for 50 kg dolphin and 4100 kg elephant.