What warning agent is added to propane after its manufacture?

The addition of ethyl mercaptan to propane serves as an essential safety measure. Propane is naturally odorless, which poses a risk as leaks can go unnoticed, potentially leading to dangerous situations such as explosions or asphyxiation. Ethyl mercaptan has a strong, distinctive odor similar to rotten eggs, which makes it easy for individuals to detect even small amounts of propane in the air. This detection capability is crucial for safety, as it prompts immediate action to address a leak.

The other options do not serve a similar purpose in propane safety. Agent P is not relevant to propane and is not recognized in this context. Ethylene glycol is primarily used as an antifreeze and does not relate to propane odorization. Green dye is typically used for marking fuels to denote tax status or type, but it does not assist in leak detection. Thus, ethyl mercaptan is the correct answer for the warning agent added to propane post-manufacture.