The correct option is A Unicellular
An organism that performs and exhibits all functions of life via a single cell, it is a unicellular organism. Such as prokaryotes like bacteria, eukaryotes like yeast are unicellular organisms. They lack a group of cells (tissue) or organ systems to perform specific functions. All the cellular functions for it to be living are performed by the individual cell. So, the correct answer is 'Unicellular'.