Final answer:
Cork is a type of plant material, not a mineral, living organism, or man-made material. Robert Hooke observed cells in a slice of cork under a microscope. These cells have thick and persistent cell walls.
Step-by-step explanation:
Cork is not a type of mineral, living organism, or a man-made material. It is a type of plant material. When Robert Hooke observed a slice of cork under a microscope, he saw what looked like small rooms, which he called cells. Cells are the smallest structural unit of living organisms, and cork cells have a unique structure with thick and persistent cell walls that outline each cell.