The correct answer is 'cell'. All living organisms are made up of cells, but if you go any smaller than a cell, what you get will no longer have the properties of life. A few properties of life include the following:
Living things grow and develop
Living things reproduce
Living things react to stimuli
Because cells do develop, split into other cells to reproduce, and the cell's nucleus is used to respond to stimuli, cells do exhibit the properties of life. However, individual parts of cells, such as cytoplasm and vacuoles, do not exhibit these properties.
Hope this helps!
