Special impact sockets and extensions are easily identified because they are:

Answer: C — Flat back

Special impact sockets and extensions are designed for heavy-duty applications and must withstand higher levels of torque and stress than standard sockets. They are often easily identified by their thicker construction, which provides increased durability and strength. This enhanced thickness allows them to absorb the impact from power tools more effectively, preventing the sockets from cracking or deforming under pressure. The thicker design is necessary because impact sockets are used with impact wrenches that deliver rapid and powerful torque. This increased thickness also contributes to a larger driving surface area, which helps to reduce the risk of rounding off fastener edges. Therefore, thick-walled construction is a key distinguishing feature that provides functional advantages in terms of performance and safety. Other features like color coding or flat backs may be present in some tools, but they are not as universally applicable as the thickness characteristic when identifying impact sockets.