The correct answer is option 3: salinity. An isohaline is an imaginary line that joins places on a map with equal salinity. Salinity refers to the concentration of salt in water.
Option 1: Radiation refers to the emission of energy as electromagnetic waves or as moving subatomic particles. It is not related to the concept of isohaline.
Option 2: Insolation refers to the amount of solar radiation reaching the Earth`s surface. It is not related to the concept of isohaline.
Option 4: Sunshine refers to the direct or indirect sunlight that reaches the Earth`s surface. It is not related to the concept of isohaline.
Therefore, the correct answer is option 3: salinity, as an isohaline is a line that connects places with equal salinity.