Which of the following admixtures is used to make concrete more flow-able?

Super-plasticizers are admixtures specifically designed to enhance the workability of concrete, making it flow more easily without requiring additional water. They increase the fluidity of the concrete mix, allowing for easier placement and finishing while maintaining or even improving the strength and durability of the finished product. This characteristic is particularly valuable in applications where intricate forms or dense reinforcement are involved, as it helps to ensure that the concrete fills all available spaces and adheres well to the reinforcement without segregation.

In contrast, air-entraining agents introduce tiny air bubbles into the concrete mix, which improves its resistance to freeze-thaw cycles but does not specifically enhance flowability. Retarders delay the setting time of concrete, making it useful in hot weather or for large placements, but they do not increase the flow characteristics. Accelerators, on the other hand, speed up the curing process, which can be beneficial in cold weather but also does not contribute to increasing flowability. Thus, super-plasticizers stand out as the correct answer for enhancing the flow of concrete.