Which substance allows direct dehydration after fixation and washing?

The correct choice is triethylphosphate, as it is a solvent that facilitates direct dehydration of tissue specimens after they have been fixed and washed. This capability makes it especially useful in histopathology, where effective dehydration is crucial for subsequent embedding and processing steps.

Triethylphosphate serves as a transitional solvent that allows for effective removal of water from biological tissues, enabling the specimens to maintain their structural integrity. Its effectiveness in this role assists in providing clear tissue sections that are essential for accurate microscopic examination.

The other substances listed do have their roles in histological processes, but they do not provide the same direct dehydration capability after fixation and washing. While isopropyl alcohol is commonly used in dehydration protocols, it typically requires a series of graded alcohol solutions to conduct a gradual dehydration process. In contrast, triethylphosphate allows for a more immediate transition.