Final answer:
Nitrogen is the safest option for leak checking because it is non-toxic, inert, and doesn't support combustion or life, making it less harmful than other gases.
Step-by-step explanation:
The gas that would cause the least damage when used for leak checking is b) Nitrogen. Nitrogen is a safe, non-toxic, non-flammable gas that makes up about 78% of the Earth's atmosphere. It is often used in leak detection because it does not react with most materials, and it is inert, meaning it doesn't support combustion or sustain life.
This makes it much safer compared to hydrogen which is highly flammable methane which is both flammable and a potent greenhouse gas and carbon dioxide which can be harmful in high concentrations and is also a greenhouse gas. Since nitrogen is widely available and has a low environmental impact it is considered the best choice for minimizing damage during leak checking procedures.