Azone of rapid temperature change between the warm surface waters and the cooler deep waters is known as the

a. thermocline

b. life zone

c. pelagic zone

d. pycnocline

A zone of rapid temperature change between the warm surface waters and the cooler deep waters is known as the A. thermocline.
Thermo refers to temperature, so this is an obvious choice. 
Life zones are areas that have similar flora and fauna. Pelagic zones refer to the open sea. Pycnocline refers to the increase of water density with depth.