Poisonous gas phosgene is formed.

When chloroform is exposed to air and sunshine, ${\mathrm{COCl}}_2$ is produced. It passes through an oxidation process, yielding Phosgene gas (${\mathrm{COCl}}_2$) and hydrochloric acid, which are used to make urethanes and polycarbonate polymers. It is maintained in dark bottles to stop any oxidation process in order to avoid this.

The chemical reaction is as follows. 

Hence , the correct option (B). Phosgene.