Final answer:
The motile portion of the sponge life cycle is the larval stage.
Step-by-step explanation:
The only part of the sponge life cycle that is motile is the larval stage. The larva is free-swimming and is covered with cilia that propel it through the water. Once the larva matures and becomes an adult sponge, it becomes sessile and spends the rest of its life attached to a substrate.