When measuring the oxygen content of the cargo tanks prior to loading cargoes requiring vapor recovery, check it __________.

Correct Answer: Option C — one half the ullage of the tank and one meter below the tank top

Under 46 CFR 39.2009, when preparing for vapor recovery, the oxygen content of the cargo tank must be measured at one-half the ullage space and one meter below the tank top. This ensures an accurate representation of the vapor space atmosphere prior to loading.