Hygroscopic substance among the following is

Options

A. calcium oxide
B. sodium oxide
C. common salt
D. calcium chloride

Explanation

To determine which of the given substances is hygroscopic, we need to understand what hygroscopic means and then analyze the options provided.

### Step-by-Step Solution:

1. **Understanding Hygroscopic Substances**:
- A hygroscopic substance is one that can absorb moisture from the air without changing its physical state. This means it can take in water vapor but does not dissolve or change into a liquid.

2. **Analyzing the Options**:
- We need to look at each option provided in the question to identify which one is hygroscopic.

3. **Option 1: Calcium Oxide (CaO)**:
- Calcium oxide is known to absorb moisture from the air and can form calcium hydroxide when it does. Therefore, it is considered hygroscopic.

4. **Option 2: Sodium Oxide (Na2O)**:
- Sodium oxide does not absorb moisture in the same way as hygroscopic substances. It reacts with water to form sodium hydroxide, which means it does change its physical state. Hence, it is not hygroscopic.

5. **Option 3: Common Salt (Sodium Chloride, NaCl)**:
- Common salt does not absorb moisture from the air and remains stable. It is not considered hygroscopic.

6. **Option 4: Calcium Chloride (CaCl2)**:
- Calcium chloride is indeed hygroscopic. It can absorb moisture from the air and can even dissolve in the absorbed water, but it is still classified as hygroscopic due to its ability to attract moisture.

7. **Conclusion**:
- Among the options provided, both Calcium Oxide (CaO) and Calcium Chloride (CaCl2) are hygroscopic. However, if we are to select only one based on the video transcript, it emphasizes Calcium Oxide as the hygroscopic substance.

### Final Answer:
The hygroscopic substance among the following is **Calcium Oxide (CaO)**.