Final answer:
Nitrogen, an inert gas, is typically used for pressure testing refrigeration systems to avoid unwanted chemical reactions or potential damage. Therefore, correct option is a.
Step-by-step explanation:
For pressure testing a refrigeration system, the chemical typically added is nitrogen (a). Nitrogen is an inert gas, which does not react with the components of the refrigeration system. Using an inert gas ensures that no undesirable chemical reactions occur that might damage the system.
Oxygen (b) is not used because it can react with oils and other substances within the system, possibly leading to combustion or other harmful reactions.
Carbon dioxide (c) could create an acidic environment when mixed with water, causing corrosion.
Helium (d) could be used for detecting very small leaks due to its small atomic size, but is not typically used for pressure testing due to its cost.