**Solid Halogen at Room Temperature**

The correct answer is option B - Fluorine.

**Explanation:**

Halogens are a group of elements in the periodic table that include fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). These elements are known for their high reactivity and tendency to form salts with metals.

At room temperature, three of the halogens exist as gases, one as a solid, and one as a liquid. Let's examine the physical states of each halogen at room temperature:

1. **Fluorine (F):**
- Fluorine is a pale yellow gas at room temperature and atmospheric pressure.
- It has a boiling point of -188.12°C (-306.62°F) and a melting point of -219.62°C (-363.32°F).
- Therefore, fluorine exists as a gas at room temperature.

2. **Chlorine (Cl):**
- Chlorine is a greenish-yellow gas at room temperature and atmospheric pressure.
- It has a boiling point of -34.04°C (-29.27°F) and a melting point of -101.5°C (-150.7°F).
- Therefore, chlorine exists as a gas at room temperature.

3. **Bromine (Br):**
- Bromine is a reddish-brown liquid at room temperature and atmospheric pressure.
- It has a boiling point of 58.8°C (137.8°F) and a melting point of -7.2°C (19°F).
- Therefore, bromine exists as a liquid at room temperature.

4. **Iodine (I):**
- Iodine is a dark gray to purple solid at room temperature and atmospheric pressure.
- It has a boiling point of 184.3°C (363.7°F) and a melting point of 113.7°C (236.7°F).
- Therefore, iodine exists as a solid at room temperature.

5. **Astatine (At):**
- Astatine is a highly radioactive element that only exists in trace amounts on Earth.
- Due to its rarity and radioactive nature, astatine's physical state at room temperature is not relevant for practical purposes.

In conclusion, among the halogens, only iodine exists as a solid at room temperature. Fluorine, chlorine, and astatine exist as gases, while bromine is a liquid.