Which factor do oceanographers determine by measuring the conductivity of a sample of seawater?

The correct answer is salinity.

Oceanographers can determine the salinity of the body of water in many ways, however, the two most usual methods are the application of salinometer or hydrometer. A hydrometer set comprises a hydrometer, a cylinder, and a temperature-salinity-density graph. The temperature is then determined and both are compared with the readings on the TSD graph. This then provides them the salinity, as salinity, temperature, and density are associated variables.

The hydrometer determines the density of water as higher salt constituent means denser water that makes the hydrometer to float higher in the cylinder. The oceanographers can also utilize a salinometer to find the salinity by calculating electrical conductivity.