The bad smelling substance formed by the action of alcoholic caustic potash on chloroform and aniline is:
A. Phenyl isocyanide
B. Nitrobenzene
C. Chloropicrin
D. Acetylene

Hint: The reaction between aniline and chloroform (\[CHC{l_3}\]) in the presence of alcoholic potash (\[KOH\]) is called the carbylamine reaction. The product of the carbylamine reaction is a bad-smelling substance.

Complete Step by Step Solution:
Primary amines (\[R - N{H_2}\]) undergo a reaction known as the carbylamine reaction (or the Hofmann isocyanide synthesis reaction) with alcoholic potash (\[KOH\]) in the presence of chloroform (\[CHC{l_3}\]). The product of the carbylamine reaction is a class of organic compounds called isocyanides or carbylamines (\[R - NC\]) that contain the isocyanide functional group (\[ - NC\]).

Image: The carbylamine reaction

Isocyanides are very well-known for their disagreeable odour, to the point that they have been investigated as potential non-lethal weapons.
The isocyanide that forms when aniline reacts with alcoholic potash and chloroform is called phenyl isocyanide (also known as isocyanobenzene). The reaction is shown below.

Image: Carbylamine reaction of aniline

Thus, the correct option is A.
Additional Information:
Alcoholic potash is responsible for converting the chloroform into dichlorocarbene which is the reactive intermediate of this reaction. The dichlorocarbene is then attacked by the nucleophilic nitrogen atom of the primary amine. Subsequent eliminations of hydrochloric acid molecules lead to the formation of carbylamine. The mechanism is shown below.

Image: Mechanism of the carbylamine reaction

Note: It is important to remember that this reaction will produce a mixture of products which will be racemic. This is because the benzyl carbocation is planar. Thus, the cyanide ion can attack it from either side of the plane. This leads to the formation of a racemic mixture of 2-phenylacetonitrile. Remember that all\[{S_N}1\] reactions lead to the racemisation of the product.

Image: Racemic mixture