Identify the reaction asked

We need the reagent that reduces an alkyne to a trans-alkene.

Recall standard reductions of alkynes

There are two important partial reductions of alkynes:

• Catalytic hydrogenation with poisoned catalyst (Lindlar catalyst) gives cis-alkene.
• Dissolving metal reduction using sodium in liquid ammonia gives trans-alkene.

Check each option

A: $\mathrm{Sn-HCl}$
This is generally used as a reducing agent for other functional groups (for example nitro groups), not for selective reduction of alkynes to trans-alkenes.
So, A is incorrect.

B: $\mathrm{H_2 - Pd/C, BaSO_4}$
This corresponds to a poisoned palladium catalyst (Lindlar-type reduction). Such reduction adds hydrogen syn, producing a cis-alkene, not trans.
So, B is incorrect.

C: $\mathrm{NaBH_4}$
Sodium borohydride reduces carbonyl compounds, but it does not reduce alkynes to alkenes under normal conditions.
So, C is incorrect.

D: $\mathrm{Na/liquid\ NH_3}$
This is the dissolving metal reduction of alkynes. It proceeds via anti addition of hydrogen and gives the trans-alkene.
So, D is correct.

Conclusion

The reagent that forms trans-alkenes from alkynes is:

$\boxed{\mathrm{Na/liquid\ NH_3}}$

Comparison with stored answer

Stored correct answer: D
Derived answer: D
Hence, they agree.