The compound which oxidizes slowly in the presence of air to give extremely poisonous phosgene gas is

Chloroform

The correct answer is Option (2) → Chloroform

The compound that slowly oxidizes in the presence of air (and light) to form the extremely poisonous gas phosgene (COCl₂) is:  Chloroform (CHCl₃)

Explanation:

• Chloroform undergoes slow oxidation:

$2\,\text{CHCl}_3 + O_2 \rightarrow 2\,\text{COCl}_2 + 2\,\text{HCl}$

• The product phosgene (COCl₂) is highly toxic.
• This is why chloroform is stored in dark bottles with ethanol (acts as a scavenger).