Answer:
The correct answer will be option D (cell wall).
Explanation:
Robert Hooke is best known for his discovery and identification of small room like structure called "cells" of plants. He discovered cells in 1665 in cork cambium of the plant which is composed of dead cells and appeared like honeycomb cavities.
Since the cells were dead so he was not able to find any cytoplasm in the cells and just observed the cell walls under the microscope. Although he discovered the cell, he was not able to discover the true function of cells.
Thus, cell wall is the correct answer.