Correct statements for the given reaction are:

A. Compound 'B' is aromatic
B. The completion of above reaction is very slow
C. 'A' shows tautomerism
D. The bond lengths of C $$-$$ C in compound B are found to be same
Choose the correct answer from the options given below.

Three molecules of ethyne polymerise at high temperature to give benzene.

$$3\,HC\equiv CH \;\xrightarrow[\text{Cu / Fe tube}]{\;873\ \text{K}\;}\; C_6H_6 + 3\,H_2$$

Let A be ethyne $$(HC\equiv CH)$$ and B be benzene $$(C_6H_6)$$. Now examine each statement.

Statement A - “Compound ‘B’ is aromatic”
Benzene has a planar cyclic ring of six sp2 carbon atoms, 6 π-electrons, and satisfies Hückel’s rule $$\left(4n+2,\;n=1\right)$$. Hence it is aromatic. Statement A is correct.

Statement B - “The completion of the above reaction is very slow”
The trimerisation of ethyne is carried out at about 873 K in a red-hot metal tube; under these drastic conditions the reaction proceeds readily. The limitation in the laboratory is removal of heat, not intrinsic slowness. Statement B is incorrect.

Statement C - “‘A’ shows tautomerism”
Prototropic (allenic) tautomerism is possible in terminal alkynes. Ethyne interconverts, though to a very small extent, with its cumulene form vinylidene:

$$HC\equiv CH \;\rightleftharpoons\; :C\!=\!CH_2$$

Because two constitutional isomers differ only by migration of a hydrogen atom along with a change in the position of a multiple bond, the phenomenon is classified as tautomerism. Statement C is correct.

Statement D - “The C-C bond lengths in compound B are found to be the same”
In benzene all six C-C bonds are equivalent owing to complete π-electron delocalisation over the ring. Experimentally each C-C distance is about 1.397 Å, intermediate between a single and a double bond. Statement D is correct.

Thus, the correct statements are A, C and D.

Option C which is: A, C and D only