ANS:B - at its boiling point

When referring to using a solvent "at its boiling point," it means heating the solvent until it reaches and maintains the temperature at which it boils under the prevailing pressure conditions. Here's how it works:

1. Boiling Point: Every substance has a characteristic temperature at which it changes from a liquid to a gas. This temperature is known as the boiling point. For water at standard atmospheric pressure (1 atm), the boiling point is 100°C (212°F). However, the boiling point can vary depending on the pressure. For example, at higher altitudes where atmospheric pressure is lower, the boiling point of water is lower.
2. Heating: To use a solvent at its boiling point, it must be heated until it reaches that temperature. For example, to use water for decoction, it is heated in a vessel until it reaches its boiling point.
3. Boiling: Once the solvent reaches its boiling point, it begins to vaporize, forming bubbles within the liquid. At this stage, the solvent is said to be boiling. During boiling, the solvent vaporizes and escapes into the atmosphere.
4. Extraction: The heat energy from the boiling solvent facilitates the extraction of soluble compounds from the solid material. For example, in decoction, the heat helps dissolve the compounds from herbs, roots, or other plant materials into the boiling water.
5. Maintaining Temperature: To ensure efficient extraction, it's important to maintain the solvent at its boiling point throughout the process. This may require adjusting the heat source to regulate the temperature or adding more solvent as it evaporates.

Using a solvent at its boiling point maximizes the extraction efficiency by providing the necessary energy to dissolve the desired compounds from the solid material. This method is commonly employed in various applications, including cooking, brewing, extraction processes in chemistry, and herbal medicine preparations.