Which of the following is a dehydrating agent used for electron microscopy?

Propylene oxide is a commonly used dehydrating agent in the preparation of samples for electron microscopy. Its role is crucial because, for electron microscopy, samples need to be thoroughly dehydrated to avoid distortion and to create a suitable environment for the imaging process. Propylene oxide is advantageous due to its ability to effectively remove water from biological specimens while preserving their ultrastructure during the embedding process in resins.

In contrast, water is a solvent that would introduce hydration rather than facilitate dehydration, making it unsuitable for this purpose. Acetic acid, while it does have a role in certain fixation processes, does not serve as an effective dehydrating agent. Isopropanol can be used for dehydration but is less common than propylene oxide in this specific context, particularly because propylene oxide has superior properties for the transition to resin embedding and is less likely to interfere with subsequent polymerization processes. Therefore, propylene oxide is the preferred choice for dehydration in electron microscopy preparations.