The correct answer is option 2: Halocline.

A halocline is a zone in the ocean where there is a sharp change in salinity with depth. It is formed due to the mixing of different layers of water with different salinity levels. In a halocline, the salinity gradient is more significant than the temperature gradient.

Option 1: Thermocline refers to the zone in the ocean where there is a rapid change in temperature with depth. While temperature does affect salinity, it is the halocline that specifically relates to salinity changes.

Option 3: The photic zone is the upper layer of the ocean where sunlight can penetrate, allowing photosynthesis to occur. It is important for the growth of phytoplankton and other marine plants.

Option 4: Pycnocline is the layer in the ocean where there is a significant change in density with depth. This change in density is caused by variations in temperature, salinity, or both. Although it can influence salinity, it is not specifically known for sharp salinity changes as the halocline.

Overall, the halocline is the specific term used to describe a zone of sharp salinity change in the vertical section of the ocean.