Final answer:
Observations that contributed to cell theory include the concepts that cells are the basic unit of life and that all cells come from pre-existing cells. Not all cells have a nucleus and cells do not spontaneously generate. Cells are fundamental components of all living organisms, not just plants. (option b)
Step-by-step explanation:
One of the key observations that contributed to the development of cell theory was the idea that cells are the basic unit of life. This concept was a fundamental aspect of the unified cell theory proposed in the late 1830s by Matthias Schleiden and Theodor Schwann. Additionally, Rudolf Virchow's observation that all cells come from other cells was crucial to this theory. Therefore, the correct answer to the question is that Cells are the basic unit of life (b) contributed to cell theory.
This discovery along with the understanding that all living organisms are composed of cells, and that all cells come from pre-existing cells, are the central tenets of cell theory. It is important to note that not all cells have a nucleus, as prokaryotic cells lack this organelle, challenging statement (a), and cells do not spontaneously generate, challenging statement (d). Moreover, the statement that only plants are made of cells (c) is incorrect because all living organisms are composed of cells.
Hence, the answer is option b