The flow of energy in an ecosystem is _______________:

Always unidirectional

The correct  answer is Option (2) - Always unidirectional

The flow of energy in an ecosystem is always unidirectional.

It is said to be unidirectional because some energy is lost in form of heat when moving from one trophic level to the next for the maintenance of the homeostasis of an organism. 

During the transfer of energy through successive trophic levels in an ecosystem, there is a loss of energy all along the path.