What is the difference between a tropism and a nastic response? A. Nastic responses require a stronger stimulus than tropisms do.
B. Nastic responses are independent of the direction of the stimulus.
C. Tropisms require a stronger stimulus than nastic responses do.
D. Tropisms are independent of the direction of the stimulus.

I think the answer is B hopefully this will help you

organisms that lack symmetry are called asymmetrical and belong to phylum porifera/sponges.

organisms with specialized stinging cells belong to phylum cnidaria. stinging cells are called cnidoblast cells.

cnidarians show two morphological forms : polyp (sessile) and medusa (mobile)

filter feeding is observed in porifera members.  

alternation of generation is observed in coelenterata or cnidaria. there is alternation of asexual polyp form and sexual medusoid form. it is also referred as metagenesis.