Give one significant difference between polyp and medusa.
a. Polyps come under cnidarians, while medusa comes under mollusks
b. Polyp is a free swimming and flat while medusa is cylindrical and attached to bottom of water bodies
c. Polyp is cylindrical and usually fixed. The medusa is umbrella like and usually free swimming
d. Polyps belong to molluscs and medusa belongs to cnidaria

Hint: The polyp form of class cnidaria is the cylindrical and sessile form. While the other forms of cnidaria i.e. medusa is umbrella shaped and free swimming.

Complete answer:
Cnidaria is a phylum under the kingdom animalia whose organisms are found in both freshwater and marine environments. They have two basic body forms called polyp and medusa.

Polyp form is a sessile life cycle of the species of phylum Cnidaria which is fixed to a substratum.

It is formed by a tube with a mouth surrounded with tentacles. Its mouth and tentacles face towards water.

Polyp forms can reproduce either sexually or asexually, furthermore, by asexual reproduction polyps produce medusa form.

Medusa is a mobile life cycle stage of species which belong to the phylum Cnidaria.

Morphologically, it is umbrella shaped and is capable of muscular contractions which help it to swim.
Medusa is capable of forming polyp forms by reproducing sexually.

Note: The Cnidarians exhibit the phenomenon of metagenesis (also called alternation of generations) is alternation between sexual and asexual phase in life cycle of an organism in which both the phases are morphologically, and sometimes chromosomally, distinct.