The correct answer is B) Oxygen meter. When gassing up a vessel, an oxygen meter is used to determine when the gassing up is complete. This is because during the gassing up process, the oxygen level in the tank or compartment being filled will decrease as the tank is filled with fuel. Once the oxygen level returns to a safe level, it indicates that the gassing up is complete. The other options are incorrect because they do not directly measure the oxygen level in the tank or compartment. A methane detector would measure the presence of methane, a CO2 meter would measure carbon dioxide levels, and a dew point meter would measure the temperature at which condensation occurs, none of which are directly relevant to determining when gassing up is complete.