1. The reaction referred to in this standard question is the keto–enol tautomerism of cyclohexane-1,3,5-trione (or a related tricarbonyl compound) giving benzene-1,3,5-triol as product.

   So we identify:

   • Compound $A$ = keto form
   • Compound $B$ = enolized aromatic form

2. Now evaluate each statement.

Statement A: Compound $B$ is aromatic

Compound $B$ is the fully enolized form with a benzene ring framework. It has a planar cyclic conjugated system with $6\pi$ electrons. Hence, by Hückel's rule, $4n+2 = 6 \quad (n=1)$ So $B$ is aromatic.

✅ A is correct.

Statement B: The completion of above reaction is very slow

Conversion of the keto form to the aromatic enol form is actually highly favored, because the product gains aromatic stabilization. Therefore, this tautomerization is not expected to be very slow in completion; rather, the aromatic form is strongly stabilized.

❌ B is incorrect.

Statement C: $A$ shows tautomerism

Compound $A$ contains carbonyl groups with adjacent $\alpha$-hydrogen(s), so it can undergo keto–enol tautomerism. Hence $A$ shows tautomerism.

✅ C is correct.

Statement D: The bond lengths of $\mathrm{C-C}$ in compound $B$ are found to be same

Since $B$ is aromatic, the carbon–carbon bonds in the ring are delocalized. Therefore, all the $\mathrm{C-C}$ bond lengths become equal, intermediate between single and double bond lengths.

✅ D is correct.

3. Therefore, the correct statements are: $A,\ C,\ D \text{ only}$

So the correct option is: $\boxed{\text{B}}$

4. Comparison with stored answer:

• Derived answer: B
• Stored correct answer: B

They match.