Gas, because it's particles are not very close to each other when compared to the other two.
The webbing on a duck's foot creates a larger surface area, which means the force of its weight is distributed over more surface area and therefore there is less pressure on the mud and its not likely to be displaced as much, so the foot doesn't get stuck.
Assuming that the collision is inelastic that is they are stuck together after the collision, momentum is conserved. We can determine the final velocity of the final mass as follows:
m1v1 + m2v2 = m3v3
100(4) + (200)(-3) = (300)v3
v3 = -0.6667 m/s
Therefore, the final mass moves at 0.6667 m/s to the left.