The correct answer is option B - Fish

Fishes have a two-chambered heart, which is the simplest structure among vertebrates. This two-chambered heart consists of:

1. Atrium (single atrium)

2. Ventricle (single ventricle)

This simple heart structure is sufficient for fish because they have:

- A relatively slow metabolism

- A simple circulatory system

- Gills for oxygen exchange, which don't require a complex heart structure

In contrast, the other options have more complex heart structures:

A. Reptiles: three-chambered heart (two atria, one ventricle)

C. Mammals: four-chambered heart (two atria, two ventricles)

D. Amphibians: three-chambered heart (two atria, one ventricle)