Which class of concrete is used in thin reinforced sections, railings, and pre-cast piles?

The correct choice refers to Class C concrete, which is specifically designed for applications that require higher strength and good durability in thin sections. This class of concrete is ideal for thin reinforced structures such as railings and pre-cast piles, where the limitations in section thickness can affect the overall structural integrity.

Class C concrete typically has a balance of high workability and strength, making it suitable for these applications where careful placement and curing are vital. It's engineered to possess the necessary physical properties that ensure that the concrete can withstand the loads and stresses associated with its use in such applications.

In contrast, the other classes of concrete may not provide the same combination of strengths and characteristics needed for thin reinforced sections. They may be formulated for different purposes, such as general construction or filling, which would not satisfy the requirements outlined in the question. Class C concrete's particular focus on performance in slender configurations helps ensure safety, functionality, and longevity in infrastructure.