3. In tropical regions, a, a permanent thermocline traps nutrients in the deep water
b. primary productivity is high all year round, because there is a constant supply of solar energy for
photosynthesis
c, the thermocline disappears in the winter
d. the upper and lower layers of water mix all year round

a, a permanent thermocline traps nutrients in the deep water

b. primary productivity is high all year round, because there is a constant supply of solar energy for  photosynthesis.

Explanation:

Thermocline occurs in the tropical regions because the temperature is very high which warms the upper layer of water bodies whereas the lower layer of water has low temperature due to no sunlight. The primary productivity is high all year in the tropical regions, because there is a constant supply of sunlight that is needed by the plants for  preparing their own food.