Cells are the smallest units that can carry on all the functions of life. They are the basic building blocks of all living organisms and are capable of reproducing, growing, responding to stimuli, and carrying out metabolic activities. Cells contain various organelles, such as the nucleus, mitochondria, and ribosomes, which perform specific functions within the cell. Molecules, on the other hand, are made up of atoms and are the smallest units of chemical compounds. Species, on the other hand, refers to a group of organisms that can interbreed and produce fertile offspring. Therefore, cells are the correct answer as they are the fundamental units of life.