Let's think step by step! Which is immobile?
Options:
- (A) adult starfish
- (B) all matter
- (C) adult goldfish
- (D) gastropod shells
- (E) tadpoles
- (F) adult sponges
- (G) an object
- (H) an organism's body
The thinking starts now:
Adult sponges are sessile. Sessile Attached permanently, immobile. Adult sponges are immobile. The final answer: (F).