How is oxygen manufactured?

Oxygen is primarily manufactured through two widely accepted processes: fractional distillation and physical separation, making the correct choice C.

In fractional distillation, air is cooled to extremely low temperatures, causing the various components of air to liquefy. This liquid air is then gradually warmed. Due to differences in boiling points, oxygen is separated from nitrogen and other gases as it vaporizes at a different temperature. This method allows for the efficient extraction of high-purity oxygen suitable for medical and industrial uses.

Physical separation methods, such as pressure swing adsorption (PSA), involve passing air through materials that selectively adsorb nitrogen, leaving mostly oxygen. This is a more energy-efficient process than liquefaction and can produce oxygen on-site for various applications.

The other options do not accurately describe the methods used for oxygen production. Combining hydrogen and nitrogen does not produce oxygen; instead, it relates to the synthesis of ammonia. Electrolysis of helium is not a standard method of oxygen production, as helium is a noble gas that does not react chemically to form oxygen. Compressing carbon dioxide would not yield oxygen, as it involves a different gaseous substance entirely. Therefore, the processes of fractional distillation or physical separation are the correct and established methods of oxygen manufacture.