The correct answer is A. Washing under pressure.

In the industrial production of hydrogen (H₂) from natural gas, carbon dioxide (CO₂) is produced as a byproduct. To remove CO₂ from the H₂ stream, a process called pressure swing adsorption (PSA) or chemical scrubbing is used.

Washing under pressure involves passing the H₂-CO₂ mixture through a solution that selectively absorbs CO₂, such as an aqueous solution of monoethanolamine (MEA) or potassium carbonate (K₂CO₃). This process is effective for removing CO₂ from the H₂ stream.