Answer :
It performs all of the functions. In order for that organism to survive, all of its needs need to be met by that one cell. If that cell isn't performing all the tasks needed to survive, the organism will die.
The given blank can be filled with performs all the function.
In case of unicellular organisms, which have a single cell, this single cell perform all the function. For a living organism, responding to stimuli, reproduction, obtaining nutrition, excretion and performing all the metabolic process is necessary. If the organism is made up of single cell, that single cell will perform all these functions necessary for life, other wise, it would be dead.
