For the given reaction: What is A (major product)?

Step 1: NaNH2 -- Dehydrohalogenation $\ce{NaNH2}$ is a strong base. It eliminates HBr:

Step 2: Red Hot Iron Tube at 873 K -- Trimerization

$3\,\ce{CH3-C≡C-CH3} \xrightarrow[873K]{Fe} \text{1,3,5-trimethylbenzene (mesitylene)}$ Trimerization of 2-butyne over red hot Fe tube gives 1,3,5-trimethylbenzene.

Key Points:

• NaNH2: strong base for dehydrohalogenation of vinyl/propargyl halides
• Fe tube at 873 K: cyclotrimerization of alkynes to aromatic compounds
• 2-butyne trimerizes to 1,3,5-trimethylbenzene (mesitylene)