A coelenterate that is commonly referred to as ‘fresh water polyp’ is

The correct option is Hydra.

To understand why this is the correct answer, let us break down the classification and features of these organisms step-by-step:

1. Understanding Coelenterates (Cnidarians):
Coelenterates (also known as Cnidarians) are a phylum of simple, radially symmetrical aquatic animals. They characteristically exist in one of two primary body forms:
- Polyp: A sessile (fixed in one place), cylindrical form (e.g., Hydra).
- Medusa: A motile (free-swimming), umbrella-shaped form (e.g., Jellyfish).

2. Comparing the Given Options:
- Obelia: This is a marine coelenterate that exhibits alternation of generations (metagenesis) between polyp and medusa stages. It is not freshwater.
- Hydra: This is a simple, freshwater coelenterate that exists exclusively in the polyp form. Because of its habitat and solitary tubular body structure, it is commonly referred to as the 'fresh water polyp'.
- Physalia: Commonly known as the 'Portuguese man-of-war', this is a marine, colonial coelenterate.
- Aurelia: Commonly known as the 'jellyfish', this is a marine coelenterate dominated by the medusa form.

Therefore, among the choices, Hydra is uniquely recognized as the freshwater polyp.