Given information:

- Molecular formula of compound A is CHNO.
- Treatment of A with Br/KOH gives an amine B.
- B responds to carbylamine test.
- Diazotisation and coupling of B with phenol give an azo dye.

To determine the structure of A, we need to analyze the given information step by step.

Step 1: Treatment of A with Br/KOH gives an amine B.

- Br/KOH is a reagent used for Hofmann's bromamide reaction.
- In this reaction, primary amides are converted to primary amines by treating with Br2 and KOH.
- The reaction proceeds via the formation of an isocyanate intermediate, which then reacts with H2O to give the amine product.
- CHNO does not contain an amide group (-CONH2) but it contains a potential precursor for Hofmann's bromamide reaction, i.e., CHNH.
- On treatment with Br/KOH, CHNH can undergo Hofmann's bromamide reaction to give a primary amine.
- Therefore, we can assume that A contains CHNH group.

Step 2: B responds to carbylamine test.

- Carbylamine test is a test for primary amines.
- Primary amines react with chloroform and alcoholic KOH to give a foul-smelling isocyanide (carbylamine) product.
- The reaction proceeds via the formation of an isocyanate intermediate, which then decomposes to give the isocyanide product.
- Since B responds to carbylamine test, it confirms that B is a primary amine.
- Therefore, the amine product obtained from the treatment of A with Br/KOH is a primary amine.

Step 3: Diazotisation and coupling of B with phenol give an azo dye.

- Diazotisation is a reaction in which a primary aromatic amine is converted to a diazonium salt by treating with nitrous acid (HNO2).
- Coupling of a diazonium salt with an aromatic compound (e.g., phenol) leads to the formation of an azo dye.
- The diazonium ion acts as an electrophile and attacks the aromatic ring of the coupling partner to form a highly coloured azo compound.
- Since B can undergo diazotisation and coupling with phenol to give an azo dye, it confirms that B is a primary aromatic amine.
- Therefore, the amine product obtained from the treatment of A with Br/KOH is a primary aromatic amine.

Step 4: Determining the structure of A.

- Based on the above analysis, we can conclude that A contains CHNH and an aromatic ring.
- The molecular formula of A is CHNO, which means it has one oxygen atom.
- Since CHNH and an aromatic ring accounts for three atoms (C, H, N), the remaining atom must be oxygen.
- Therefore, A can be represented as CHNH-C6H5O, which simplifies to CHCONH.
- Hence, the correct option is B.

I willl suggest to read carbylamine test again.