**Explanation:**

**Dry Cleaning Process:**
Dry cleaning is a process of cleaning clothes without using water. Instead, it uses a solvent, typically a petroleum-based chemical called perchloroethylene (also known as perc). Dry cleaning is suitable for delicate fabrics such as silk and wool, as it helps to prevent shrinking, fading, and damage that can occur with traditional washing methods.

**The Role of Petrochemicals:**
Petrochemicals, specifically perchloroethylene (perc), are the primary solvent used in dry cleaning. Perc is a highly effective cleaning agent that can remove stains and dirt from fabrics without causing damage. It has a low boiling point, which allows it to evaporate quickly, leaving the clothes dry and ready to wear.

**Silk and Woolen Clothes:**
Silk and woolen clothes are delicate and require special care when cleaning. These fabrics can easily shrink, lose their shape, or become damaged if exposed to water or harsh detergents. Dry cleaning with petrochemical solvents like perc ensures that silk and woolen clothes are cleaned thoroughly without the risk of damage.

**Hard Water vs. Soft Water:**
Hard water contains high levels of dissolved minerals such as calcium and magnesium ions. Soft water, on the other hand, has low mineral content and is treated to remove impurities. When it comes to dry cleaning, the type of water used is not relevant because the process does not involve water at all. Instead, it relies on the solvent, which is a petrochemical.

**Conclusion:**
In dry cleaning stores, silk and woolen clothes are washed using petrochemical solvents like perchloroethylene (perc). This method of cleaning is suitable for delicate fabrics and ensures that the clothes are thoroughly cleaned without the risk of damage. The use of petrochemical solvents eliminates the need for water, making the type of water (hard or soft) irrelevant in the dry cleaning process. Therefore, the correct answer to the question is option 'C' - petrochemicals.