The part of the cell theory that best explains Robert Hooke's observations is:
- All living organisms are composed of cells.
This part of the cell theory explains Hooke's discovery of "cells" in cork because it states that all living things, including plants, animals, and microscopic organisms, are made up of cells. Hooke observed the box-shaped structures in cork and other specimens, which led to the understanding that cells are the basic units of structure and function in living organisms. This observation aligns with the concept that all living things are composed of cells, supporting the cell theory proposed by scientists like Hooke.