Consider the following reactions: A is:

A gives precipitate with Ag2O

$\ce{Ag2O}$ (or $\ce{AgNO3/NH3}$) gives a white/light yellow precipitate with terminal alkynes only (acidic sp C-H reacts to form silver acetylide). So A = terminal alkyne. Eliminates 2-butyne (d) and ethene (c).

Key Points:

• Terminal alkynes give precipitate with $\ce{Ag2O}$
• Propyne + $\ce{Hg^{2+}/H^+}$ $\rightarrow$ acetone (Markovnikov, methyl ketone)
• NaBH4 reduces acetone $\rightarrow$ isopropanol (secondary alcohol)
• Lucas test: secondary alcohol gives turbidity within 5 min