Which describes the missing step?
Oxygen is removed from the water before step 3.
Oxygen is removed from the water after step 3.
Tentacles sting and paralyze prey after step 1.
Tentacles sting and paralyze prey before step 1.

The missing step is oxygen is removed from the water after step 3.

Explanation:

The steps listed by Xavier are:

Porifera or sponges are aquatic, immobile invertebrates whose structure is saccular with an upper opening, the osculus.

Sponges absorb water through their pores, where the porocytes are found, and through flagellated cells -the coanocytes- the movement that allows the entrance of water to the spongocele or atrium is produced, in addition to the phagocytosis of organic elements.

Once the water is in the interior chamber of the sponges, the organic particles are phagocyted to be digested, and the oxygen is removed from the water, and passes through diffusion into the cell.  The waste products, as well as carbon dioxide, are released into the water that will be expelled through the osculus.

In Xavier's list, the missing step is the removal of oxygen from the water, before it is expelled from the body of the sponge.

Learn more:

Sponge structure link