Fiber-reinforced concrete is a composite material that consists of concrete and steel fibers. The primary purpose of adding steel fibers to concrete is to improve its mechanical properties, such as tensile strength, toughness, and resistance to cracking.

A common shape of steel fibers used in fiber-reinforced concrete is the hooked-end shape. This particular design is beneficial for several reasons:

1. Improved Bond Strength: The hooked ends of the fibers help in anchoring with the concrete matrix, thereby increasing the bond strength between the fibers and the concrete.
2. Enhanced Pull-out Resistance: Hooked-end fibers provide greater pull-out resistance compared to straight fibers. This characteristic is crucial during concrete cracking, as it contributes to the ductility and post-crack behavior.
3. Better Distribution: Hooked-end fibers are more effective in distributing loads across the concrete, thus reducing the chance of localized cracking.

To analyze why the other options are not suitable:

• Cylindrical: While fibers may be generally cylindrical in shape, this option does not specify an end configuration like 'hooked.' Therefore, it doesn't clearly denote the advantageous shape used for reinforcement purposes.
• Cubic: This shape is not practical for steel fibers as it doesn't provide the necessary length and form to reinforce concrete effectively.
• Round: Similar to cylindrical, 'round' does not describe the end design, which is vital for enhancing the bond and performance of the fibers.

Thus, the correct answer is Hooked-end, which effectively meets the mechanical performance criteria needed in fiber-reinforced concrete.