Which of the following materials is considered fire-retardant?

Gypsum board, also known as drywall, is considered a fire-retardant material due to its composition. It is made from gypsum, a mineral that is non-combustible and has the ability to resist heat. When exposed to fire, gypsum board releases water vapor in the form of steam, which helps to slow down the spread of fire. This inherent property is why it is commonly used as a fire barrier or in fire-rated assemblies in buildings, helping to protect structural integrity and slow the progression of flames.

In contrast, untreated wood is combustible, cotton fabric generally ignites easily, and most types of plastic sheeting can melt and contribute to the spread of flames. Therefore, the fire-retardant qualities of gypsum board make it the correct choice among the options provided.