$$2-$$Hexyne gives trans$$-2-$$Hexene on treatment with :

An alkyne can be converted to an alkene by partial hydrogenation. The stereochemistry (cis / trans) of the product depends on the reagent used.

Case 1: Catalytic hydrogenation over Pd, Pt, Ni or Lindlar’s catalyst
  • Reagents such as $$Pt/H_2$$ or $$Pd/BaSO_4\;( \text{Lindlar’s catalyst})$$ add molecular hydrogen to the alkyne by a syn mechanism.
  • Both hydrogen atoms approach the triple bond from the same side, giving the cis (Z) or syn-alkene.
  • Example: $$RC \equiv CR \xrightarrow[\text{H}_2]{\text{Lindlar}} cis\text{-}RCH\!=\!CHR$$.

Case 2: Dissolving-metal reduction in liquid ammonia
  • Metals such as Na or Li in liquid $$NH_3$$ (−78 °C) generate solvated electrons. These electrons reduce the alkyne by an anti mechanism.
  • Hydrogen (from $$NH_3$$) adds to opposite faces of the π-system, producing the trans (E) alkene.
  • Example: $$RC \equiv CR \xrightarrow[\text{NH}_3]{\text{Na or Li}} trans\text{-}RCH\!=\!CHR$$.

Case 3: Other reagents in the list
  • $$LiAlH_4$$ is a hydride donor; it reduces alkyl halides, carbonyls, etc., but it does not reduce an isolated carbon-carbon triple bond.
  • Therefore it leaves $$2$$-hexyne unchanged.

Applying these facts to $$2$$-hexyne, the desired product is trans-$$2$$-hexene. Only the dissolving-metal reduction (Li in liquid $$NH_3$$) provides this anti addition.

Hence, the correct reagent is

Option B which is: $$Li/NH_3$$