Which reagent is best suited to convert an internal alkyne into a trans-alkene?

Solution

1. Reduction of alkynes with $\text{Na}$ in liquid ${\text{NH}}_3$ proceeds via a radical anion intermediate.
2. The trans-configuration is favored due to the repulsion between the alkyl groups in the intermediate.
3. This method is specifically used for the synthesis of trans-alkenes.
4. Other reagents like Lindlar catalyst produce cis-alkenes.
5. Hence the answer is (C).