The correct answer is OB. All its cells.
In a unicellular organism, all its cells work together to carry out its life functions. The cell is both the structural unit and functional unit of life, and a single cell is capable of performing all the activities necessary for life. This includes processes such as obtaining energy, obtaining nutrients, eliminating waste, reproducing, and responding to stimuli. As a result, all the cells in a unicellular organism play a critical role in its survival and are essential for carrying out its life functions.