\(CCl_4\) is a well-known fire extinguisher. However, after using it to extinguish fire, the room should be well ventilated. This is because

it produces phosgene by reaction with water vapours at higher temperatures

The correct answer is option 3.  it produces phosgene by reaction with water vapours at higher temperatures.

Carbon tetrachloride (\( \text{CCl}_4 \)) is used as a fire extinguisher because of its ability to form a heavy vapor that smothers flames. However, it poses significant health hazards, especially when exposed to high temperatures. Here's a detailed explanation of why the room should be well ventilated after using \( \text{CCl}_4 \) to extinguish a fire:

Production of Phosgene Gas: At high temperatures, \( \text{CCl}_4 \) can decompose, especially in the presence of water vapor. The reaction that occurs can produce phosgene gas (\( \text{COCl}_2 \)), which is highly toxic and was used as a chemical warfare agent during World War I. The chemical reaction can be summarized as follows:

\(\text{CCl}_4 + H_2O \overset{\text{high T}}{\longrightarrow} \text{COCl}_2 + 2HCl\)

In this reaction, carbon tetrachloride reacts with water vapor to produce phosgene and hydrochloric acid (HCl).

Toxicity of Phosgene: Phosgene gas is extremely hazardous. It can cause severe respiratory damage, irritation to the eyes and skin, and can be fatal even at low concentrations. Symptoms of phosgene exposure might include coughing, choking, and severe respiratory distress, which can sometimes be delayed.

Immediate Ventilation: To mitigate the risk of inhaling these toxic fumes, it is crucial to ventilate the area immediately after using \( \text{CCl}_4 \) as a fire extinguisher. This helps to disperse any phosgene gas that may have formed and reduces the concentration of \( \text{CCl}_4 \) vapors, minimizing the risk of poisoning.

Given these hazards, the correct and primary reason for ventilating the room after using \( \text{CCl}_4 \) is (3) it produces phosgene by reaction with water vapours at higher temperatures.