Salt bridge contains

Options

A. calomel
B. corrosive sublimate
C. `H_(2)O`
D. agar-agar paste

Explanation

To answer the question "Salt bridge contains," we need to analyze the options provided and understand the role of a salt bridge in electrochemistry.

### Step-by-Step Solution:

1. **Understanding Salt Bridge**: A salt bridge is a device used in electrochemical cells to maintain electrical neutrality by allowing the flow of ions between the two half-cells. It typically contains an electrolyte solution.

2. **Analyzing the Options**:
- **Option A: Calomel** - Calomel refers to mercury(I) chloride, which is not used in salt bridges.
- **Option B: Corrosive Sublimate** - This is another name for mercuric chloride, which is also not suitable for salt bridges.
- **Option C: H2O** - While water is a solvent, it does not serve as an effective medium for a salt bridge on its own.
- **Option D: Agar Agar Paste** - This is a gelatinous substance derived from algae and is commonly used in conjunction with potassium chloride (KCl) as an electrolyte in salt bridges.

3. **Conclusion**: Given the properties of the options, agar agar paste is the correct choice as it is a semi-solid medium that can hold the KCl electrolyte without dissociating into ions, thus maintaining the integrity of the salt bridge.

### Final Answer:
**The correct answer is Option D: Agar Agar Paste.**