The correct answer is B. Accelerators.
Accelerators are admixtures that cause early setting and hardening of concrete. They are used when early strength is required, such as in cold weather or when concrete must be placed under water. Accelerators work by increasing the rate of hydration of the cement, which is the chemical reaction that causes concrete to harden.
Workability admixtures are used to improve the workability of concrete, which is its ability to be placed and compacted. They do this by reducing the water demand of the concrete, which makes it easier to work with.
Retarders are admixtures that slow down the setting and hardening of concrete. They are used when long working times are required, such as in hot weather or when concrete must be transported over long distances. Retarders work by slowing down the rate of hydration of the cement.
Air entraining agents are admixtures that introduce small air bubbles into concrete. These air bubbles improve the workability of concrete and make it more resistant to freezing and thawing.