The liquid which causes formation of poisonous gas when exposed to sunlight in presence of air or oxygen \((O_2)\) is:

\(CHCl_3\)

The correct answer is option 2. \(CHCl_3\).

When chloroform (\(CHCl_3\)) is exposed to sunlight in the presence of air or oxygen, it can undergo a photochemical reaction known as photolysis. In this process, the ultraviolet (UV) radiation from sunlight provides the energy needed to break chemical bonds in chloroform molecules.

One of the possible reactions that can occur during the photolysis of chloroform involves the formation of phosgene gas (\(COCl_2\)). This reaction proceeds through a series of intermediate steps, but the overall reaction can be represented as follows:

\[ 2 CHCl_3 + O_2 \rightarrow 2 COCl_2 + 2 HCl \]

The phosgene gas produced in this reaction is highly toxic and can pose serious health hazards. Phosgene was used as a chemical warfare agent during World War I, highlighting its extreme toxicity.

Therefore, when chloroform (\(CHCl_3\)) is exposed to sunlight in the presence of air or oxygen, it can indeed produce poisonous gas, making option 2 the correct choice.