Steel is the most elastic among the options provided, denoted by option [C]. Elasticity refers to the ability of a material to regain its original shape after deformation. Steel exhibits high elasticity, allowing it to stretch and bend under stress but return to its original form once the stress is removed. This property makes steel an ideal material for various structural applications, including bridges, buildings, and machinery components, where resilience and durability are essential. While rubber also displays elasticity, its elasticity is typically lower than that of steel. Wet soil and plastic have limited elasticity compared to steel and rubber. Therefore, in terms of elasticity, steel stands out as the most elastic material among the options listed, offering superior resilience and flexibility in various engineering and construction applications.