Which is a part of the cell theory? a)cells come from pre-existing cells b)cells have a nucleus c)organisms with one cell must be seen with a microscope d)organisms must have more than one cell to live

The first statement is a part of the cell theory, cells come from pre-existing cells. Not all cells have a nucleus, all the prokaryotes lack nuclei.
There are one-celled organisms and some types of cells can be so small that they need super magnifying microscopes in order to be observed (more powerful than the regular light microscopes) 

Hhhdhdns