Which of the following compounds can be detected by Molisch's test?

Molisch’s test is a general laboratory test used to detect the presence of carbohydrates (sugars).

Principle of the test
• A small amount of the sample is treated with a few drops of alcoholic α-naphthol (Molisch reagent).
• Concentrated $$H_2SO_4$$ is then carefully added along the side of the test tube so that it forms a separate lower layer.
• If carbohydrates are present, they undergo dehydration by the acid to form furfural (from pentoses) or hydroxymethyl-furfural (from hexoses).
• These furfural derivatives condense with α-naphthol to give a violet-purple ring at the interface of the two layers. The appearance of this ring constitutes a positive Molisch’s test.

Now examine each option:

Option A - Nitro compounds: They do not form furfural derivatives under these conditions, so they give no violet ring. Molisch’s test is negative.

Option B - Sugars (carbohydrates): They are dehydrated by conc. $$H_2SO_4$$ to furfural/hydroxymethyl-furfural, which reacts with α-naphthol to give the characteristic violet ring. Molisch’s test is positive.

Option C - Amines: Amines do not undergo the required dehydration; hence they do not respond to Molisch’s test.

Option D - Primary alcohols: Ordinary primary alcohols (e.g., ethanol) are not carbohydrates; they also fail to produce the furfural derivative, so the test is negative.

Therefore, the only compound class that can be detected by Molisch’s test is sugars.

Option B which is: Sugars