Ephemeral art is artwork that is intentionally temporary or made from materials that naturally deteriorate, melt, decay, or disappear over time. The defining characteristic is that the work does not endure permanently.
An ice sculpture is a clear example of ephemeral art because it is carved from ice, which inevitably melts at temperatures above freezing. The artwork exists only for a limited time before the material returns to water and the sculpture is lost. This transience is an inherent property of the medium.
A mosaic is made from durable materials such as stone, glass, or ceramic tiles set in morite or cement - mosaics can survive for thousands of years, as seen in ancient Roman and Byzantine examples. A mural is a painting applied directly to a wall or ceiling surface; while murals can deteriorate over time, they are intended to be permanent and can last for centuries with proper care. A bronze statue is cast from bronze, one of the most durable sculptural materials, resistant to corrosion and capable of lasting millennia.
Of the four options, only the ice sculpture is made from a material that guarantees the work will be short-lived, making it ephemeral by nature.