The following is absent in air .

Options

A. hellium
B. radon
C. neon
D. argon

Answer

To solve the question "The following is absent in air," we need to analyze the options provided: helium, radon, neon, and argon.

### Step-by-Step Solution:

1. **Identify the Elements**:
- The elements given in the options are:
- A) Helium (He)
- B) Radon (Rn)
- C) Neon (Ne)
- D) Argon (Ar)

2. **Research Presence in Air**:
- **Helium**: Helium is a noble gas and is present in trace amounts in the atmosphere.
- **Radon**: Radon is a radioactive noble gas that is not typically found in the atmosphere due to its radioactivity and short half-life.
- **Neon**: Neon is also a noble gas and is present in the atmosphere in trace amounts.
- **Argon**: Argon is one of the major components of air, making up about 0.93% of the atmosphere.

3. **Determine Which is Absent**:
- From the analysis, we see that:
- Helium, Neon, and Argon are present in the atmosphere.
- Radon, due to its radioactive nature and instability, is not found in significant amounts in the atmosphere.

4. **Conclusion**:
- Therefore, the element that is absent in air is **Radon (Option B)**.

### Final Answer:
**Radon (Option B) is absent in air.**

---