Detergent has a common group

Options

A. `RC_6 H_4 SO_3 Na`
B. `R Ona`
C. `RCOONa`
D. `RCO_2 ONa`

Explanation

### Step-by-Step Solution:

1. **Understanding Detergents**: Detergents are synthetic cleaning agents that are designed to remove dirt and grease from surfaces. They differ from traditional soaps in their chemical composition and functionality.

2. **Common Group in Detergents**: The common group found in most detergents is the sulfonate group. This is represented by the chemical structure RC6H4SO3Na, where:
- R represents an alkyl group (a hydrocarbon chain).
- C6H4 indicates a phenyl group (a benzene ring).
- SO3Na is the sulfonate group, which is the sodium salt of sulfonic acid.

3. **Role of Sulfonic Acid**: Sulfonic acid (SO3H) is crucial in the formulation of detergents. It allows detergents to function effectively in various types of water, including hard water, which is rich in calcium and magnesium ions.

4. **Comparison with Soaps**: Unlike soaps, which are made from carboxylic acids and do not lather well in hard water, detergents containing the sulfonate group can produce lather in all water types. This is due to the non-biodegradable nature of the sulfonate group, which helps in maintaining the detergent's effectiveness.

5. **Conclusion**: Therefore, the common group in detergents is the sulfonate group, specifically represented as RC6H4SO3Na.

### Final Answer:
The common group in detergents is RC6H4SO3Na.

---