Experimentally reducing a functional group cannot be done by which one of the following reagents?

Step 1: Identifying Reducing Agents

• Pd-C/$\ce{H2}$ and Pt-C/$\ce{H2}$: Standard catalysts for heterogeneous hydrogenation of double bonds, triple bonds, and nitro groups.
• Zn/$\ce{H2O}$: Can reduce specific groups (like alkyl halides or in Clemmensen reduction context with $\ce{HCl}$).
• $\ce{Na/H2}$: This is not a standard reagent. Sodium is used with alcohol (Birch reduction or Bouveault-Blanc) but not combined directly with gaseous hydrogen for standard reductions.

$\boxed{\text{Answer: (a) Na/H2}}$