A unicellular organism has only one cell, such as bacteria, protists, and yeast. A multicellular organisms is consisted of 2 or more cells, such as tissue organs, and humans.
Modern cell theory is the theory that cells are the basic structural and organizational of both single and multi cell organisms. This is when cells divide, pass on hereditary info., and contain energy that flows through them.
Sorry I don’t understand the third question.
Humans are multicellular organisms, heterotrophic, and reproduce through sexual relations. Hope this helped :)
