Which of the following is concentration cell

Options

A. `Cu_((s))//Cu_((aq.1M))^(2+)||||Cu_((aq.1M))^(2+)//Cu_((s))`
B. `Cu_((s))//Cu_((aq.0.5M))^(2+)////Cu_((aq.0.5M))^(2+)//Cu_((s))`
C. `Zn_((s))//Zn_((aq.0.5M))^(2+)//Cu_((aq.1M))^(2+)//Cu_((s))`
D. `.^(Theta)Pt//H_(2_("g 1 bar"))//HCl_(aq" "0.002M)////HCl_((aq.0.005M))//H_(2(g.1"bar"))//Pt^(o+)`

Explanation

To determine which of the given options is a concentration cell, we need to understand the definition and characteristics of a concentration cell.

### Step-by-Step Solution:

1. **Definition of a Concentration Cell**: A concentration cell is an electrochemical cell that consists of two half-cells made of the same material but with different concentrations of the same electrolyte. The difference in concentration creates a potential difference, allowing the cell to generate electricity.

2. **Analyzing the Options**:
- **Option 1**: Both half-cells have the same concentration (0.5 M). Since the concentrations are identical, this cannot be a concentration cell.
- **Option 2**: The concentrations are different (0.002 M and 0.005 M), but the electrolytes are different. For a concentration cell, the electrolytes must be the same. Therefore, this option does not qualify as a concentration cell.
- **Option 3**: Similar to option 2, if the electrolytes are different, it cannot be a concentration cell, even if the concentrations differ.
- **Option 4**: This option has the same electrolyte but different concentrations. For example, if one half-cell has a concentration of 0.002 M and the other has 0.005 M of the same electrolyte, this qualifies as a concentration cell.

3. **Conclusion**: Based on the analysis, the correct answer is **Option 4**, as it meets the criteria of having the same electrolyte with different concentrations.