Define frequency:

the distance from the rest position to a crest or a trough; the more energy, the greater the amplitude

a repeated back-and-forth or up-and-down motion.

the part of a longitudinal wave where the particles are far apart

the number of waves that pass a set point in a certain amount of time

the number of waves that pass a set point in a certain amount of time

Explanation:

Frequency (f) is the number of cycles a wave can complete in one second. The standard unit of frequency is Hertz (Hz).

Mathematically, frequency of a wave is inversely proportional to its period (T)

i.e F = 1/T

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.