The vessels a and b of equal volume and weight are immersed in water to a depth h. The vessel a has an opening at the bottom through which water can enter. If the work done in immersing a and b are Wa and Wb respectively, then

To understand the relationship between the work done in immersing the two vessels, let us analyze the physical processes involved for both vessel a and vessel b.

1. Immersing Vessel b (Closed Vessel):
Vessel b is completely closed (or has no opening for water to enter). As it is pushed down into the water to a depth $h$, it displaces a constant volume of water equal to its own volume (once fully submerged). The upward buoyant force acting on it is maximum and remains constant (or increases to its maximum value as it gets fully submerged). To immerse it, work $W_b$ must be done against this full buoyant force.

2. Immersing Vessel a (Vessel with an opening at the bottom):
Vessel a has an opening at the bottom. As it is immersed in water, water enters the vessel through the opening. This means the air inside is either compressed or escapes, and the space inside the vessel gets filled with water. Since water enters the vessel, the net volume of water displaced by vessel a is significantly less than that of vessel b.
Because the volume of water displaced by vessel a is smaller, the upward buoyant force acting on vessel a during the immersion process is much less than the buoyant force acting on vessel b.

3. Comparing the Work Done:
Work done is equal to the integral of the force required to submerge the vessel over the depth $h$.
Since the buoyant force opposing the immersion of vessel a is less than the buoyant force opposing vessel b at every stage, the force required to push vessel a down is less than that for vessel b.
Therefore, the work done in immersing vessel a is less than the work done in immersing vessel b:

$W_a<W_b$

Thus, the correct option is Wa < Wb.