For the given reaction:

What is A?

The reaction mentioned in the question belongs to the class of cyclic trimerisation of alkynes. In such reactions, three molecules of a terminal alkyne combine in the presence of catalysts like $$Cu_2Cl_2/NH_4Cl$$, $$AlCl_3$$, $$Fe$$ (red-hot tube) etc. to give a substituted benzene.

General rule for terminal alkyne $$RC \equiv CH$$:
Three molecules cyclise to give a benzene ring in which the group $$R$$ appears at the 1,3,5-positions (meta positions):
$$3\,RC \equiv CH \;\xrightarrow{\text{catalyst}}\; C_6H_3R_3$$

In the question, the alkyne used is propyne: $$CH_3C \equiv CH$$, where $$R = CH_3$$.

Applying the rule:
$$3\,CH_3C \equiv CH \;\xrightarrow{\text{trimerisation}}\; C_6H_3(CH_3)_3$$

The product formed is therefore $$1,3,5\text{-trimethylbenzene}$$, commonly called mesitylene.

Matching with the given options:
Option B: 1,3,5-trimethylbenzene  ⟶  correct.

Hence, $$\displaystyle A = \text{1,3,5-trimethylbenzene}$$.