When welding with an oxy-acetylene outfit, __________.

Correct Answer: Option D — open the acetylene cylinder valve only 1/4 to 1/2 turn and leave the wrench on the valve stem

Opening the acetylene cylinder valve only 1/4 to 1/2 turn allows for rapid emergency closure if a fire occurs. Keeping the wrench on the valve stem ensures immediate access to shut off the gas supply without searching for tools, a critical safety practice for preventing catastrophic cylinder fires.