Maximum heat transfer in heat exchanger happend in :
a) turbulant
b) counter current
c) co-current

b) Counter current

Explanation:

In mechanical, chemical, nuclear and other systems, it happens that heat must be transferred from one place to another or from one fluid to another. Heat exchangers are the devices that allow you to perform this task  the types of exchangers are presented  of heat as a function of flow: parallel flow; counterflow; cross flow.

Among the main reasons why exchangers are used

Heat are as follows:

• Heat a cold fluid using a fluid with a higher temperature.

• Reduce the temperature of a fluid by means of a fluid with a lower temperature.

• Bring the fluid to the boiling point using a fluid with a higher temperature.

• Condense a fluid in a gaseous state by means of a cold fluid

A backflow occurs when the two fluids flow in the same direction but in  opposite way. Each of the fluids enters the exchanger through different ends Since the fluid with less  temperature goes backflow from the heat exchanger at the end where the fluid enters with higher temperature,  the temperature of the coldest fluid will approach the temperature of the inlet fluid.

This type of exchanger  turns out to be more efficient than the other two types mentioned above. In contract with the exchanger  parallel flow heat, the counterflow exchanger may have the highest temperature in the cold fluid and the  lower temperature in the hot fluid after heat transfer in the exchanger.

Be careful with turbulent that it is not a type of exchanger but a system in which a fluid is found.

catya your not at all which answer i've seen you do this on another question!