Compound A is converted to B on reaction with CHCl$$_3$$ and KOH. The compound B is toxic and can be decomposed by C. A, B and C respectively are:

The reaction described is the carbylamine reaction (isocyanide test): when a primary amine is treated with CHCl₃ and KOH (alcoholic), it gives an isocyanide (isonitrile), which has a characteristic foul smell and is highly toxic. The reaction is: R-NH₂ + CHCl₃ + 3KOH → R-N≡C: + 3KCl + 3H₂O.

This reaction is specific to primary amines — secondary and tertiary amines do not give this positive test. Therefore, compound A must be a primary amine.

Compound B is the isonitrile (isocyanide) formed, which is indeed toxic. This is R-N≡C (isocyanide / isonitrile), not a nitrile (which would be R-C≡N).

The isonitrile B can be destroyed (decomposed / hydrolyzed) by heating with concentrated HCl: R-N≡C + HCl + H₂O → R-NH₂ + HCO₂H (or HCOOH / formic acid). This converts the toxic isonitrile back to a primary amine and formic acid.

Thus: A is a primary amine, B is an isonitrile compound, and C is concentrated HCl. This corresponds to Option 3.