Absolute alcohol can be prepared from rectified spirit by: A. Azeotropic distillation with benzene. B. Fractional distillation. C. Keeping over fresh CaO for a few hours and then distilling. D. Distillation under reduced pressure.

To prepare absolute alcohol from rectified spirit, which typically contains around 95% alcohol, you need to remove the remaining water content. Among the options provided:

A. Azeotropic distillation with benzene: This method is used to separate azeotropic mixtures, but it's not typically employed for producing absolute alcohol.

B. Fractional distillation: Fractional distillation can be used to separate components with different boiling points. While it's a common method for alcohol purification, it may not yield absolute alcohol directly.

C. Keeping over fresh CaO for a few hours and then distilling: This method involves using calcium oxide (CaO) to absorb water from the alcohol, followed by distillation. This is a common method for preparing absolute alcohol, as calcium oxide is a strong desiccant.

D. Distillation under reduced pressure: Distillation under reduced pressure lowers the boiling point of the components, allowing for separation at lower temperatures. While it can be used in alcohol purification, it's not specifically for producing absolute alcohol.

The correct answer is therefore C. Keeping over fresh CaO for a few hours and then distilling.